GB2084041A - Process and apparatus for wastewater treatment - Google Patents

Abstract

A process and apparatus for removing suspended and colloidal materials, including greases and oils, from previously untreated wastewater which contains domestic sewage, comprising clarification or screening without chemical addition followed by granular medium filtration without chemical addition to produce filtrate containing less than 50 percent of untreated wastewater suspended solids. The granular medium filter is intermittently backwashed with filtrate and surfactant and/or oxidizing agent. Air is intermittently passed upward through the medium to reduce flow resistance and to inhibit anaerobic biological activity.

Description

SPECIFICATION Process and apparatus for wastewater treatment This application is a continuation-in-part of Application Serial No. 190,605, filed September25, 1980.

The present invention relates to a wastewater treatment process and to the apparatus for accomplishing such process. More specifically, the present invention relates to an improved method of treating wastewater which enhances the separation, concentration and treatment of poorly settleable and colloidal solids by fine sand filtration, thereby increasing the energy generation potential of the treatment process and reducing or eliminating subsequent biological treatment requirements. The present invention reduces capital expenditures and related energy costs.

Solids contained in wastewater may be either suspended or dissolved. They may be filterable or unfilterable, volatile or inert, and may or may not be biodegradable. These solids, as well as greases and oils typically present in wastewater, generally represent the most significant pollutant constituents, and it is these deleterious materials that must be either removed or treated in the wastewater treatment process.

The suspended solids range in size from 108 microns to 100 microns and larger. Generally, solids 10 microns and larger are settleable and may be removed by gravity separation. Solids ranging from 103 micron to 10 microns normally will not settle, and generally require coagulation and flocculation for removal. Solids 103 microns and smaller are generally considered to be dissolved solids.

Most biological sewage treatment systems and processes are designed to provide for separation of the large insoluble particles and for biological treatment of the particles that will not settle. Many of these systems are subject to erratic performance due to general inflexibility of the prior separation process, i.e. primary clarification and or screening.

Primary processes may be subject to highly variable hydraulic and/or organic loading, resulting in an effluent of varying quality. The biological systems subsequently employed must operate within this variation in load which may minimize the efficiency of the biological system.

The suspended and colloidal solids not separated by either screening or clarification must further be treated by one or more biological systems in order to meet secondary treatment levels. The energy, capital, and maintenance costs attendant to those systems are substantial.

Two such generally accepted biological systems are the suspended growth modifications of the activated sludge process, and the fixed growth system modifications of the trickling filter process. These systems can vary from rather crude to highly sophisticated processes depending on the size of the plant and equipment involved. The requirement of effluent nitrification will further complicate the process with additional capital equipment and energy costs.

Many attempts have been made to modify the treatment processes in order to reduce the energy costs of biological systems. This has been generally attempted by modification of the process and design changes of various mechanical components in order to improve the solids separation efficiency of the clarifier. An accepted and effective solids separation technique has been the addition of chemical coagulant to the influent wastewater which enters the gravity settling device or clarifier. This technique materially increases the quantity of solids separated, and reduces the organic load to the biological system or subsequent treatment systems.

This reduction in load reduces the amount of energy required by the biological processes.

Physical-chemical processes, that have been recommended for the treatment of wastewater, generally contemplate the addition of a coagulant and the flocculation of suspended particles. This is generally performed either in a zone within the clarifier, or prior to the clarifier, to improve settling of the suspended solids. The clarified wastewaterthen may be treated in a number of ways, such as by adsorbing onto activated carbon or even refiltering followed by carbon adsorption.

However, the addition of a chemical flocculant and/or a coagulant adds chemical costs and creates very serious problems, in many instances, far grea terthan the advantages obtained by the reducing of the suspended solids discharged to the biological system.

The precipitation of the coagulant generates a chemical sludge which can be very difficult and very expensive to treat and dewater. Further, the chemical sludge generally adversely affects the generation of sludge gas.

Organic material treated biologically in an activated sludge system will generate new and additional cellular material. This highly structured cellular material ultimately creates a potential secondary polluting load, and therefore, must be carefully separated and disposed of in a manner that will not affect the environment. Therefore, in most biological systems, the activated sludge or trickling filter humus, both consisting in major part of new cellular material, must be separated from the wastewater.

This is generally accomplished by secondary clarifiers. The settled sludge may be returned to a digester for further biological reduction. The digested sludge is ultimately withdrawn and dewatered by various means. The digestion, the dewatering and the disposal of the new cellular materials are difficult and expensive processes.

Granular media filters are currently used to remove suspended solids particles from wastewaters following other treatment steps. One such filter effectively used in such applications is described in U.S. Patent Reissue No.28,458, issued July 1, 1975.

Improvements are shown in U.S. Patent No.

3,817,378, issued June 18, 1974 and U.S. Patent No.

3,840,117, issued October 8, 1974.

An effective method and apparatus for cleaning grease and oil from the media of granular media filters is described in U.S. Patent No. 4,032,443, issued June 1977.

An apparatus and method for separating and con centrating solids from filter backwash liquid is disclosed in U.S. Patent No. 3,792,773.

The present invention is intended to overcome many of the disadvantages of conventional systems of wastewater treatment by removing and collecting organic solids from wastewater containing domestic sewage without the use of coagulant chemicals. A major improvement is the additional energy potential of the increased quantity of chemical-free primary sludge. Another major advantage is a corresponding reduction in production of new cellular plant material.

The present invention is a wastewatertreatment process for the removal of suspended and/or colloidal materials, including possibly greases and oils, from previously untreated wastewater which contains domestic sewage, comprising the steps of: a. passing said wastewatercontaining domestic sewage to a clarifier or screening device to remove a portion of the materials and clarify said wastewater, without essential addition of chemical; b. passing clarified wastewater without essential addition of chemical through a granular medium filter within a filter tank to an underdrain cavity to pro ducefiltrate containing less than 40 percent of the suspended solids present in the untreated wastewater;; c. intermittently passing air upwardly through the granular medium to reduce resistance to flow through the medium and inhibit anaerobic biological activity therein, without disturbing the filter medium integrity; and d. intermittently backwashing said filter with a portion of said filtrate and a surfactant and/or oxidizing agent to remove accumulated deleterious materials.

Thus, wastewater containing domestic sewage from a device such as a screen, seive and/or primary clarifier is passed through a granular medium filter without prior biological, chemical or physicalchemical treatment. Alternatively, the solids removal device may be eliminated entirely; wastewater is passed directly to the inlet of the filter and nonsettleable and colloidal solids are trapped by the filter medium and adsorbed by the filter medium surface.

The effectiveness of the removal of these nonsettleable solids is a function of the application rate and the physical characteristic of the medium, typically sand. Generally, at least 60 percent of the suspended material is removed by a single filter. Solids are allowed to accumulate in the filter medium until the resistance through the filter medium causes the water level above the medium to reach a predetermined level, at which time the filter is backwashed, and the deleterious materials previously trapped and adsorbed are scrubbed and washed from the medium.

In accordance with the invention, there is also provided an improvement in the interception and storage of colloidal and suspended particles by periodically reducing the resistance of the filtering medium, body and surface by passing air upwardly through the medium intermittently to improve the porosity and to retard anaerobic decomposition. This air is motivated upwardly by filtrate rising with in the underdrain cavity and displacing the air. The upward movement of air is sufficient to move deleterious materials into the interior of the filter bed where such materials are temporarily stored, without disturbing the integrity of the filter medium.

With another aspect of the present invention, there is provided a method of backwashing, utilizing vertical hydraulic jets intermittently with air to provide high velocity agitation and medium movement at a variable subfluidized rate to improve the effective ness of the backwashing system and utilize a minimum of backwash water.

In accordance with another aspect of the present invention, the backwashed water is directed to a holding tank where the previously nonsettleable particles, now in the backwash water, agglomerate and settle. Greases and oils are also permitted to agglomerate for skimming. The settled solids are allowed to concentrate generally without the addition of chemicals. In accordance with another aspect of the present invention, the backwashed solids are pumped to a digester for anaerobic stabilization of the organic material, and biological generation of a methane and carbon dioxide fuel gas.

In accordance with another aspect of the present invention, the clarified backwashed water may be directed back to the inlet of the filter for refiltering and under some conditions, may be directed to a receiving stream or water course such as a lake, river, ocean, etc.

The polluting characteristics of the filtrate are materially reduced by the removal of the colloidal and nonsettleable solids present in the clarified wastewater.

In accordance with another aspect of the present invention, oxygen is added to the liquid body by impingement, by diffuser and by pulsing through the filtering medium, in order to minimize the normal oxygen deficiency of clarified waste and counter anaerobic conditions in the medium and liquid filter body.

In accordance with another aspect of the invention, there is provided an improved medium regeneration cycle that removes grease, oil and biota attached to the medium surface by emulsification and the hydraulic and pneumatically induced agitation of the medium bed. Additionally, an oxidizing agent such as hypochlorite with or without a surfactant is used to regenerate the medium surfaces. Preferably, however, a non-halogen oxidizing agent such as hydrogen peroxide, oxygen containing gas or liquid, or ozone is used either with or without the detergent or surfactant, thus avoiding the creation of chlorinated hydrocarbons.

Another major aspect of this invention is the staging ofthefiltration processes in orderto increase the quantity of unsettled and colloidal solids removed from the liquid waste. Typically, in a two-stage filter system, the secondary filtrate contains less than 25 percent of the suspended solids in the untreated water, and the accompanying low BOD. Such filtrate may generally be discharged directly to a water course, without further biological treatment.

A further aspect of this invention is the use of very small quantities of coagulant between the staging of the filter processes, in order to further reduce the very fine solids that will neither be trapped nor adsorbed to the medium surface. The coagulation chemical is rapidly mixed with the primary filtrate and passed to the second filter without a flocculation step. Such a process will typically result in a secondary filtrate having less than 20 mg/1 suspended solids and may contain less BOD than normally attributed to secondary processes. The first stage filter removes the largest portion of solids, and only very small quantities of coagulant are required in the second stage. This the effect of such chemical upon subsequent sludge treatment is negligible.

Yet another object of the present invention is the use of a filter to treat excess flows that may adversely affect an existing or fixed design biological system by bypassing excess flows around the biological system through a fine medium filter and remove sufficient quantities of suspended solids from this bypassed wastewater, as well as from the biologically treated wastewater. This permits the filtering of the treated and untreated wastewater to meet effluent requirements. In this invention the efficiency of the biological process is maintained.

The biological process is protected against hydraulic overload, and overall performance is ensured.

A further aspect of this invention is the combination of two stage filtration within a single vessel providing for varying head conditions relative to quantities of flow and designed to accept a portion of extremely high flows through more coarse filtering media.

The two stage filter provides the opportunity for more than one sand size and varying application rates for a given filtering load.

Another aspect of this invention is the scrubbing of the vertical walls of the filter tank on an automatic select basis during a backwash. A cleaning agent such as a detergent and oxidizing agent or alternately a surfactant and an oxidizing agent may be added to the backwash waterto further clean the walls. Cleaning agent addition may be controlled manually or on an automatic select basis. A nonhalogen oxidizing agent is preferred.

Another aspect of this invention is the addition of an oxidizing or sterilizing liquid to the backwash liquid at the end of the backwash cycle in orderto control biota growth. This sterilizing or oxidizing liquid will also cause the oxidation, emulsification or removal of the previously attached organic substances on the medium surfaces resulting from the filtration of the clarified waste through the fine sand medium. The medium surfaces are effectively utilized to adsorb some of the organic components that make up soluble BOD.

Another aspect of this invention is the ability to minimize the deleterious effect of the colloidal and suspended particles which plug the pores of activated carbon and coat the carbon surfaces with organic substances that limit the effective use of activated carbon.

Another aspect of this invention is an improved underdrain system that effectively distributes the oxidizing or sterilizing liquid into the filter medium without premature mixing with backwash liquid and detergent or surfactant.

#Figure 1 is a schematic line drawing of a conventional waste water treatment system, employing as major elements a primary clarifier, a biological treatment system, a final clarifier and a sludge digester.

Figure 2 is a schematic line drawing of the preferred embodiment of the invention, illustrating the many alternate treatment systems that may be utilized.

Figure 3 is a schematic line drawing of the invention, combined with biological treatment.

Figure 4 is a schematic line drawing of the invention, combined with a biological treatment system followed by a second filter.

Figure 5 is a schematic of a physical-chemical treatment process placed between two filters.

Figure 6 is a schematic showing application of the invention to a waste treatment system subject to high variations in influent flows, illustrating treatment approaches for bypassed flows.

Figure 7 is a sectional view of the invention utilized in treating bypassed flows.

Figure 8 is a sectional view of the invention illustrating the wall washing and pulsed bed system.

Figure 9 is a sectional view of the underdrain support system described in this invention.

Figure 10 is a sectional detail of an outlet nozzle of the underdrain support system described in this invention.

Figure 11 is a sectional view of an alternate of the nozzle detail of the underdrain support system described in this invention.

Reductions of total biochemical oxygen demand (BOD), suspended solids and soluble BOD of clarified domestic wastewater by parallel granular medium filters are illustrated in Table I, in which the filters utilized granular media consisting of quartz or 0.35 mm or 0.45 mm effective size, respectively, 10 inch (25.4 cm) sand depth and application rates of 80 liters per minute per square meter (2 gpm/ft2).

Tablel Filter Filter Filter #1 #2 In fluent Effluent Effluent Sand Size, mm 0.35 0.45 Suspended Solids, mg/1 81.3 20.8 25.7 Removal Efficiency% (74.6) (68.6) Turbidity FTU 46 23 24 TotalBOD5,mg/1 131.5 49.0 53.9 Removal Efficiency% (62.7) (59.0) Soluble BOD8, mg/1 35.3 23.3 24.9 Removal Efficiency % (34.0) (29.3) Average Filter Run Length, hr. 3.0 6.0 Backwash to Filtrate Ratio, 1/1 0.168 0.088 A significant factor generally overlooked in the design and development of waste treatment systems is that the majority of the organic polluting load in wastewater, either shown as Theoretical Oxygen Demand (TOD), Chemical Oxygen Demand (COD), or Biochemical Oxygen Demand (BOD), is contained in the nonsettleable suspended and colloidal solids.

Such solids are present in domestic sewage. For example, wastewater solids that have been separated by size have the organic impact shown in Table II. Basically, 66 percent of the organic polluting load is contained in the suspended solids of particle size 8 microns and smaller as measured by COD. It is generally conceded that suspended solids 8 microns and smaller are not settleable by themselves.

The significance of the polluting load of clarified or settled wastewater may be further illustrated by a size and weight analysis of a clarified wastewater sample. The sample of Table II was drawn from the Lorain Ohio Waste Water Treatment Plant, and is not untypical of clarified wastewater which contains domestic sewage. The wastewater sample was then filtered progressively through increasingly smaller pore membrane filters. The quantity of solids retained on each filter was then determined, as well as the chemical oxygen demand (COD) for the solids retained.

Table II Lorain Ohio Primary Effluent Settled Wastewater TSS 84.5 mug/1; COD 182 mg/1 Filter pore TSS retained by COD passed through size, um Filter, mgl 1 Filter, mug/7 8.00 38.4 121.0 3.00 14.7 80.9 1.20 10.7 62.7 .80 11.5 62.7 .45 7.6 65.9 .22 1.6 48.9 The reduction of soluble BOD is another aspect of this invention. The reduction of soluble BOD by fine sand filtration is quite significant, and becomes even more substantial at lower hydraulic application rates. For example, tests have shown that the reduction in soluble BOD ranges from a high of 34 percent reduction, to a low of 5.6 percent. Soluble BOD is a definition rather than a true description.Colloidal particles that can pass through a 0.45 micron filter are calculated to be part of the soluble fraction which are measured as soluble BOD.

A substantial quantity of the colloidal material that does pass through a 0.45 micron membrane filter will attach itself and be adsorbed to the grain surface in a granular medium filter. Some of the organics in the solution are also rapidly biologically converted to biomass as wastewater is passed through a granular bed. The quantity removed in a function of the application rate, and size and depth of the sand.

Table Ill, delineating a specific series of tests, illustrates the effectiveness of the removal of soluble BOD by the present invention. Therefore, single or multi-stage filtering not only removes the measurable non-settleable solids, but materially reduces the soluble fraction of the organic load.

Table Ill Soluble BOD, Reduction (Percent) Flow rate M35mm 0.45mm 2 GPM/FT2 34 29.5 5 GPM/FT2 18.3 15 8 GPM/FT2 7.2 5.6 Referring now to the drawings, wherein the showings are for the purpose of illustrating the preferred embodiments of the invention only, and not for the purpose of limiting same.

Referring to Figure 1, a conventional, biological waste treatment process is delineated. The raw wastewater inlet conduit 10 is directed to the comminutor or screening device 20, and through a conduit 11 to the primary settler or primary clarifier 100.

The clarified liquid is directed through conduit 110 to the biological process 400, wherein the waste is biologically treated, generating new cellular material, and is directed through conduit 410 to the final settling tank 500. A major portion of the sludge solids is settled in clarifier 500 and withdrawn through conduit 520. A portion of the sludge is recycled to the biological process via conduit 521. The excess or waste sludge then mixes with the sludge that has been withdrawn from primary clarifier 100 through conduit 120 and the admixture of the waste sludge and the primary sludge is directed through conduit 121 to digester 800, wherein the sludge is further treated, and a portion of the organics is converted to methane and carbon dioxide.The excess waste sludge from clarifier 500 may be passed directly to digester 800, and avoid the mixing process in con duit 121, depending upon the Engineer's preference.

Figure 2 shows a schematic block diagram of the present invention, wherein a filter system 200 is placed following a primary settling tank or clarifier 100. The wastewater treatment system includes an inlet wastewater conduit 10, wherein wastewater is directed to comminuting and/or coarse screening device 20, then directed by conduit 11 to a primary clarifier 100. The clarified effluent leaves the primary clarifier 100 through conduit 110 to the inlet of filter 200.

As shown in Figure 2, the filtrate in conduit 210 is directed to one or more of subsequent elements of treatment, such as biological treatment, which may be either the suspended growth or attached growth system. Alternately the filtration may be followed by second stage filtration, and then treated by activated carbon or biological treatment for removal of organic matter and/or nitrogen, or discharged to a receiving stream, or some other means of treatment or reuse. The necessity for the optional second stage filtration is a function of wastewater quality and particle size distribution, as well as governmental discharge requirements. In many cases the load to the biological system is so reduced that adequate biological treatment is obtained, for example, in a trickling filter without any recirculation.

Another embodiment of this invention is a wastewater treatment process for the removal of suspended and colloidal materials, including greases and oils, proteinaceous compounds and ammonia nitrogen from previously untreated wastewater which contains domestic sewage, comprising: a. passing said wastewatercontaining domestic sewage to a clarifier or screening device to remove a portion of the materials and clarify said wastewater, without addition of chemical; b. passing clarified wastewaterwithout addition of chemical through a first granularfilter within a first filter tank to a first underdrain cavity, to produce a primary filtrate;; c. passing said primary filtrate through a fixed media filter such as a trickling filter having nitrifying microorganisms growing on said fixed media, to oxidize said nitrogen in primaryfiltrateto nitrate and nitrite; d. passing nitrified primary filtrate through a second granular medium filter with a second filter tank to a second underdrain cavity, to produce secondary filtrate containing less than 20 mg/1 suspended solids and 20 mg/l BOD8; e. intermittently passing air upwardly through each granular medium to reduce resistance to flow through the media, and inhibit anaerobic biological activity therein; f. intermittently backwashing said fi rst fi Iter with a portion of said primary filtrate and a surfactant and/or oxidizing agent to remove accumulated deleterious material;; g. intermittently backwashing said second filter with a portion of said secondary filtrate and a surfactant and/or oxidizing agent to remove accumulated deleterious material.

Such treatment readily produces a secondary filtrate containing less than 20 mg/I suspended solids and 20 mg/I BO D, even when the fixed media filter is operated at a rate equal to or exceeding 0.34 GPM per square foot, without any recirculation. The secondary filtrate will contain less than 1.5 mg/I NH4-N if the wastewatertemperature exceeds 68 F.

The backwash from filter 200 is discharged through conduit 220 to clarifier 300, wherein the backwashed solids are permitted to settle, and the clarifier supernatant is returned to the inlet of filter 200 through conduit 310 and filter inlet conduit 110.

The quality of the supernatant may be enhanced by the addition of coagulation chemicals. The settled sludge from clarifier 300 is withdrawn through conduit 320 and directed to the sludge withdrawal line 122 which transports sludge from the primary clarifier 100 through conduit 120.

The filtrate quality will depend on the rate of wastewater application, wastewater pollutant concentrations, the bed depth and medium grain size utilized in filter 200. The filtrate quality can be further enhanced by a series of subsequent treatment options, depending upon the ultimate water quality requirement.

The energy requirement for subsequent treatment of clarified wastewater will be materially reduced as a result of the reduction in organic polluting load of the clarified wastewater accomplished by filter 200.

Additionally, the sludge created by the concentrated solids in the backwash water from filter 200 is added to the sludge removed in the primary settling tank or clarifier 100. The total quantity of primary sludge is the sum of the primary sludge developed by both the gravity clarifier 100, and filter 200. The fact that no coagulant is added to the inlet of either the primary settling tank or the filter, ensures that the total sludge withdrawn shall contain a minimum of treatment chemicals.

Anaerobic biological production of fuel gas methane from sewage sludge is known to proceed more readily and efficiently when the ratio of primary to secondary biological sludge is high. This invention results in materially increased primary solids and significantly reduced secondary solids quantities.

For example, a typical domestic wastewater contains 160 mg/I of suspended solids, one half of which is removed by simple primary treatment without chemicals.

The clarified wastewater typically has the following analysis: Suspended Solids 80 mg/l BOD5 130 mg/l In standard treatment, the clarified wastewater is subjected to activated sludge treatment to meet secondary governmental effluent standards. Approximately 70 mg/l of biological sludge solids will be produced. Thus the biological solids comprise nearly one half of the total solids load which normally is anaerobically digested.

In the present invention the clarified wastewater is filtered through a granular medium to meet the secondary standards, thus replacing the biological step which has high equipment and operating costs.

Additionally, no biological treatment solids are passed to the anaerobic digester; therefore gas pro duction and solids reduction as well as subsequent solids dewatering are all enhanced.

Where the biological treatment step is preceded by granular medium filtration, the greatly reduced quantity of new cellular material results in much improved digester performance.

Referring further to Figure 2, conduit 151 following second stage filtration may direct the filtrate to an activated carbon contactqr. Filtrate may alternately be treated in a biological system as further shown in Figure 3, or discharged to a stream or reused. Alternately, second stage filtration may be omitted and single stage filtrate be directed to the named alternate processes.

A major advantage of the invention is the reduction of the organic polluting load to all of the subsequent treatment systems by filter 200, thereby reducing the energy and capital requirements.

Referring now to Figure 3, wherein the same type of secondary biological process is illustrated as in Figure 1, the secondary treatment plant process may now be modified by the use of a filter installed in the clarified effluent conduit 110. The filtrate contained in outlet 210 requires significantly reduced biological treatment. Biologically treated wastes may be directed to a final settler or clarifier 500, or directed to subsequent filter 700 as shown in Figure 4, and then to the chlorine contact tank 600.

Also, in accordance with this invention, the prim ary sludge is withdrawn from the primary settling tank 100 through conduit 120. The generally nonset- tleable and colloidal solids in the clarified wastewater are intercepted then by filter 200, the filtrate from filter 200 being then directed through conduit 210 to the biological process 400. The backwashed water in conduit 220 is directed to the inlet of backwash separator 300, wherein the backwashed solids are allowed to settle, and are discharged and mixed with the solids of the primary settling tank 100, thus increasing the total primary solids removed from the process.

The organic load, based on the greater removal of organic solids, is materially reduced to the biological process. Depending again on the application and medium size, the residual organic load to the biological process can be reduced to the point that the new cell growth may be effectively intercepted by a second filter of generally the same type as utilized in filter 200 and a final settler is not needed. The filter 700 will then intercept the admixture of solids and new cells and return those solids to the inlet of the biological process 400 through conduit 721 as shown in Figure 4. Excess waste solids are then discharged to the backwash separator 300 th rough conduit 722.

In accordance with this invention, the potential for growth of new cellular material has been limited by the reduction in suspended and non-settleable matter by granular medium filtration. The growth of new cells is proportional to the organic loading in the wastewater entering the biological process through conduit 210.

For example, assuming a new cell growth rate of 0.4 lb. new cells per I b. BOD removed, a filtrate residual containing 50 mg/l of BOD, would generate only 20 mg/l of new cells. Wastewaterwith suspended solids level of less than 30 mg/l of solids and proportionate values of biochemical oxygen demand may be classified as secondary effluent. In the event that the suspended solids are in excess of the permitted stream requirements of the particular water course, a second filter may be added and the 20 mg/l of new cells may easily be separated out by the filter700, and the filtrate through conduit 710 will meet a low solids stream requirement. Should stream requirements be such that the solids to be discharged are not in excess of the requirement, no additional treatment may be required.

Now referring to Figure 5, settled primary effluent is directed to the first stage filter 200 and the filtrate directed through conduit 210 to mixing chamber 30 where very small concentrations of coagulant from vessel 40 are pumped into mixing chamber 30 through conduit 50. Coagulant is mixed in mixing chamber 30 with first-stage filtrate from filter 200 and the coagulant4iltrate admixture discharged to filter 700 through conduit 211, without a flocculation step. The coagulant-filtrate admixture is then filtered through filter 700, and the second-stage filtrate discharged through conduit 710. The backwash liquid from the first stage filter 200 is discharged to the backwash holding tank 300 through conduit 220, and the backwash from filter 700 is also directed to the same backwash solids separating device through conduit 720.The settled sludge in vessel 300 is discharged through conduit 320 and mixed with the sludge removed from the primary vessel 100 through conduit 120, and the combined sludge volume is directed to the digester 800 through conduit 123.

The effluent from conduit 710 may then be treated optionally in a number of ways in accordance with the requirements of the ultimate use or disposition of the treated wastewater. Water that is refiltered may be directed as shown in Figure 2 to a full series of further treatment steps depending on ultimate use and requirement. This type of system greatly reduces or completely eliminates the biological cel iulargrowth material normally associated with biological treatment plants, and at the same time provides a high quality effluent. The limitation, of course, will be the soluble constituents now not removable by filtration. Wastewater effluent, from which the suspended solids and colloidal solids have been removed by filtration, is readily amenable to contact with activated carbon for the adsorption of residual organics. This type of pretreatment can effectively save tremendous space and energy and provide an effluent of controlled quality. This is especially true in the many areas of water reuse. The filtering process will reduce the energy requirements normally associated with conventional wastewater treatment providing similar performance.

Referring now to Figure 6, the primary filtration approach may be applied most successfully to wastewater treatment systems that are subjected to extremely high variations in flows, such as during storms, wherein the existing biological processes are threatened by a hydraulic overload. Now referring specifically to the drawing Figure 6, influent conduit 10 represents the inflow conduit of the wastewater, including variable volumes of storm water passing through a comminuting or screening device 20, then through an alternate flow diversion chamber and excess flow weir 60 into the primary settling chamber 100. The clarified effluent in conduit 110 is directed to hydraulic control chamber 80, wherein the first stage excess flow will be directed around the biological process 400 and the final clarifier 500 into conduit 510 which is the influent to filter 200.Backwash water is directed from filter 200 to conduit 220 to settling vessel 300. The supernatant from settling vessel 300 is returned to conduit 510 through conduit 310. The settled sludge is returned through conduit 320 and mixed with the settled sludge from clarifier 100 through conduit 120 and directed for further treatment through conduit 124 to a digester or other sludge treatment process.

In accordance with this invention, the excess flows are directed around the biological process to protect the integrity of the biological process from upset by hydraulic impact, and further reduce the biochemical oxygen demand of the bypassed flow passing through conduit 90, by the action of filter 200 to reduce the suspended solids.

Referring again to Figure 6, excess flows may also be directed around the primary clarifier 100, in the event designs indicate that maximum hydraulic conditions to clarifier 100 would be exceeded. Therefore, a diverting chamber 60 may be installed ahead of primary clarifier 100 and the flows beyond design values may be directed through conduit 70 into bypass conduit 91 to conduit 510 into filter 200 for further improvement through the removal of suspended solids by filtration.

Another aspect of this invention is the ability of the filtering device to meet variable filtrate requirements as the influent volumes change due to the impact of storm water flow and dry weather flow. During dry weather, when wastewaterflows are low, the residence times of solids in the sewer lines is relatively long, resulting in considerable solubilization of pollutant material. Wastewater BOD, is high.

On the other hand, high water levels during wet weather flush the sewer lines quickly. The wastewater is not only diluted by the extra water but is more "fresh", e.g., less solubilization has occurred.

While high flows hinder operation of conventional biological treatment systems, the pollutant materials in such high4lowwastewaters are more amendable to removal by granular medium filtration than are pollutants in low-flow wastewaters. As a result, bypassing excess flows of wastewater around biological treatment systems to a granular medium filterwill maintain a high biological treatment level while providing a high level offiltrationtreatmenttothe excess flows. The result is a highly acceptable effluent at a time when the effluent quality is normally deteriorated.

In another embodiment, the once-through granu lar medium filter of Figure 7 provides for effective and efficient treatment of wastewater under highly variable flow rates. Referring to Figure 7, the treated and bypassed influent flows are directed to filter vessel 200 through conduit 510 as previously illustrated in Figure 6. Referring back to Figure 7, a flow indicator 530 located in conduit 510 signals the quantity of flow to the filter. Within preset limits and at the lower levels of the range, valve 512 would remain open and valve 514 remain closed. The dry weather flow at low water levels from conduit 510 enters filter 200 at a low hydraulic head level over the filter media 250. As the flows increase, flow meter 530 would then signal valves 512 to close and valve 514 to open, and allow the water level to enter filter 200 at a higher point.

Simultaneously, flow indicator 530 would change the operating level from the previous operating water level indicated at level 240, to level 242, and eventually to 244, thus increasing the capacity of the filter. As the flow further increases, a signal from flow indicator 530 would then limit the flow through flow diversion box 516 to filter 200 and cause a proportionate flow to pass through conduit 518 into filter vessel 200A located entirely in the confines of filter vessel 200. The filtrates from filter 200 and filter 200A passing through conduits 223 and 223A are then blended together into a single conduit for plant effluent. The operating levels within filter 200, in turn, are changed proportional to the flow demand as indicated in control by flow indicator 530.

Referring again to Figure 7, backwash trough 260 is located overthe filter sand 250, and prior to backwash, the water level 240 is lowered to the level of the backwash trough weir 262. In the prior art, this volume of water over the filter trough weir 262 is added to the backwash water volume, thereby increasing the amount of backwash water to be treated.

As the operating water level changes as directed by flow indicator to 242 or 244 orto intermediate levels, the volume of wasted water is, of course, increased.

One aspect of this invention is the placement of filter cell 200A entirely within the confines of filter 200 thus reducing the volume of water over the filter bed 150 without reducing the efficiency of the filter system. The volume of wasted water is thus reduced.

Another aspect of this invention is the dumping of the volume of water over trough weir 262 into s separate dump chamber 270 prior to the start of the backwash pump 222, thus greatly reducing the volume of backwash water to be treated. The sequence of backwash operation is initiated by a backwash requirement signal from level sensor 352, 354, or 356 proportionate to the appropriate water level 240, 242, or 244 as controlled by the signal from flow indicator 530. Upon signal, valve 277 opens and allows the body of liquid over the trough weir 262 to drain through line 266 to wastewaterdump chamber 270 wherein the total volume of liquid over the weir 262 is now retained. After the liquid body overweir 262 is drained as provided by a signal from a timing device, valve 277 is closed and valve 278 is opened, backwash pump 222 is then energized, valve 226 is opened, valve 225 is closed, and the backwash liquid is then directed into the underdrain cavity 208 and through the sand bed 2#50 to the overflow weir 262.

The backwash liquid is then directed to the back wash holding tank 290 and may be further treated as described in my U.S. Patent 3,792,773. The liquid volume now retained in tank 270 is pumped into filter 200 by pump 272 after pump 222 has been shut down, valve 226 is closed and valve 225 has opened, and the filtrate from filter 200 is again directed to the filtrate holding tank "clearwell" 280.

The volume of liquid over weir 262 contains materially less solids than the backwash liquid. The mixing ofthese volumes increases the total liquid volume and increases the cost of treating that liquid volume by diluting the backwash admixture. This dilution will reduce the ability of the admixture to reach the point of auto flocculation without the addition of chemicals. Auto flocculation or agglomeration proceeds much more readily as an increased concentration of solids.

The filter chamber head water dumping, shown as part of the filter system in Figure 7 with other aspects of this invention, may be used independently. This aspect of reduced volume of head water results in an improvement in operating costs and efficiencies in any system of filtration where volumes of water are stored over the level of the backwash weir.

By modification of the piping, this filter may also be used to treat wastewater in two consecutive stages. In this case, wastewater is first passed through the upper filter bed 250A and then through filter bed 250. Granular medium in the first bed 250A will be relatively coarse while bed 250 will be composed offinergranules.

Now, referring to Figure 8, another feature of filter 200 is illustrated. Perimeter washing ring 226 is placed at the upper wall of filter 200 with nozzles 228 placed to scour the walls of the filter 200. Washing ring supply conduit 224 communicates with backwash pump 222 and chemical feed pump 262 through conduit 266. One aspect of this invention is the utilization of a specialty cleansing solution to backwash the walls of the filter at preselected periods. The filtering of primary waste contemplates the introduction of highly contaminated biological wastes into the filter cell. Biological wastes will coat the walls, whereon biota may grow. In turn, the absence of oxygen will cause objectionable growths and odors. Effective cleansing and disinfection of the walls is necessary to improve the total operation of the filter system.

Another aspect of this invention is the improved backwashing system for removal of the particles that are tenaceiously attached to the medium grains and the introduction of cleansing and disinfecting solution into the medium at preselected periods to retard biota growth.

Now referring to Figure 8 and specifically drawing attention to the backwash technique, a high pressure air system is alternately provided through air pressure pump 280, directing air through conduit 282 to static mixer 284, mixing air and backwash water and directing this admixture to the underdrain cavity 208.

The air intermittently fed into the static mixer 284 causes an additional agitation of the medium 250, due to the pulsating vertical rising jets 292 in the underdrain system. Thus the backwash more thoroughly scours the medium, than has been pre viously possible, and minimizes the need for fre quent chemical cleaning. However, the same system is designed so the chemical may be fed into the underdrain system from chemical holding tank 260 to adequately disinfect the filtering media, retarding a biota build up in eitherthe medium orfiltersurface.

The most serious problem associated with the filtering process of wastewater is the adsorption and accumulation of colloidal grease and/or films to the surcace of the medium grains. This particular prob lem is magnified by the high percentage of colloidal grease and oil present in primary or combined storm and primary flows. Contact with and backwashing with a detergent solution or oxidant and surfactant effectively removes colloidal grease and oil from the surface of the medium grains.

Another aspect of this invention is thus an improvement in the underdrain system described in my U.S. Patent 3,840,117 as well as an improvement in the chemical clean system described in my U.S.

Patent 4,032,443. Frequent or excessive chemical clean cycles utilizing halogenated compounds can be detrimental to the operation of a biological system or the quality of filtrate of the wastewater treat ment system. Halogenated compounds such as those containing bromine or chlorine can combine with hydrocarbons present in the wastewaterto form trihalomethanes which have been character ized by the Environmental Protection Agency as car cinogens. Bromoforms and chloroforms are exam ples of halogenated hydrocarbons that may result from the use of halogenated compounds.

Now referring to Figure 9, a preferred embodiment of this aspect of the invention, is shown. An improvement in the underdrain system described in my patent 3,840,117 is illustrated, including the mod ification by the addition of oxidant chamber 1000, inlet conduit 1100, and compound sleeves 1200.

Chamber 1000 is formed of an upper plate and a lower plate, generally coextensive with and support ing a network of horizontal bars. The bars, in turn support the screen, and the screen supports the granular medium bed. The plates are separated in parallel relationship by non-limiting spacers.

Uniformly spaced sleeves 1200 extend vertically through both upper and lower plates for allowing a liquid or gas flow either upward or downward.

Uniformly spaced air distribution chambers 208 are art open at the lower end and closed at their upper end by the lower oxidant chamber plate.

A detergent, generally a linear alkyl sulfonate is introduced into the underdrain. After a premeasured quantity of detergent is introduced into the under drain cavity, an oxidant solution such as of hydrogen peroxide, or a gaseous oxidant such as ozone is fed into conduit 1100, into chamber 1000, into nozzle 1400 through orifice 1420, and mixed with detergent and filtrate solution forced up into air distribution chamber 208 and through orifice 1450. Alternate construction of the sleeves 1200 and location of orifices 1450 are shown in Figures 10 and 11.

Detergents or surfactants such as L.A.S. are biodegradable, and oxidizing solutions as strong as hydrogen peroxide or ozone for example will reduce the strength of the admixture if they are mixed prior to entry into the granular medium, and permitted to remain in a condition mixed prior to use in the medium. This apparatus protects the strength of the chemicals and the use of a biodegradable detergent with a strong oxidant by mixing them just prior to, at, or just following passage of filtrate into the medium, yet provides a uniform distribution into the medium of the admixture. This method reduces the cost of chemical cleaning, eliminating formation of anytrihalomethanes and problems in disposing of non-biodegradable or phosphate type detergent.

The chemical clean system will rapidly emulsify any residual grease coating on the medium grains, and wash the surfaces of the grains free of any grease or oil residual.

Another aspect of this invention as shown in Figure 9 is the ultimate passage of air into the underdrain system through conduit 1500 and intermittently operated valve 1510 into conduit 1100, which causes a jet pulsation. This jet pulsation more effectively removes the grease and oil film from the sand medium.

Claims (28)

1. A wastewatertreatment process for the removal of suspended and/or colloidal materials, including possibly greases and oils, from previously untreated wastewater which contains domestic sewage, comprising the steps of: a. passing said wastewater containing domestic sewage to a clarifier or screening device to remove a portion of the materials and clarify said wastewater, without essential addition of chemical; b. passing clarified wastewaterwithout essential addition of chemical through a granular medium filterwithin a filter tank to an underdrain cavity to produce filtrate containing less than 40 percent of the suspended solids present in said untreated wastewater;; c. intermittently passing air upwardly through the granular medium to reduce resistance to flow through the medium and inhibit anaerobic biological activity therein, without disturbing the filter medium integrity; and d. intermittently backwashing said filter with a portion of said filtrate and a surfactant and/or oxidizing agent to remove accumulated deleterious materials.

2. A wastewatertreatment process for the removal of suspended and/or colloidal materials, including possibly greases and oils, from previously untreated wastewater which contains domestic sewage, comprising the steps of: a. passing said wastewater containing domestic sewage to a clarifier or screening device to remove a portion of the materials and clarify said wastewater, without essential addition of chemical; b. passing clarified wastewater without essential addition of chemical through a first granular medium filter within a first filtertank to produce a primary filtrate; c. passing said primary filtrate without essential addition of chemical through a second granular medium filter in a second filter tank to produce secondary filtrate containing less than 25 percent of the suspended solids present in said untreated wastewater;; d. intermittently passing air upwardly through the granular medium of each filterto reduce resistance to flow through the medium and inhibit anaerobic biological activity therein, without disturbing the filter medium integrity; e. intermittently backwashing said first filter with a portion of primary filtrate and a surfactant and/or oxidizing agent to remove deleterious materials; and f. intermittently backwashing said second filter with a portion of secondary filtrate and a surfactant and/or oxidizing agent to remove deleterious materials.

3. Awastewatertreatment process according to claim 2, wherein the effective medium grain size of said second filter is smaller than the effective medium grain size of said first filter.

4. Awastewatertreatment process according to claim 2 or 3, wherein said secondary filtrate contains less than 30 mg/I suspended solids and 30 mg/I BOD5.

5. A wastewatertreatment process according to any one of claims 2-4, wherein the medium of said second filter is activated carbon.

6. A wastewatertreatment process for the removal of suspended and/or colloidal materials, including possibly greases and oils, from previously untreated wastewater which contains domestic sewage, comprising the steps of: a. passing said wastewater containing domestic sewage to a clarifier or screening device to remove a portion of the materials and clarify said wastewater, without essential addition of chemical; b. passing clarified wastewater without essential addition of chemical through a first granular medium filter within a first filter tank to a first underdrain cavity to produce a primary filtrate;; c. rapidly mixing a coagulation chemical with said primary filtrate and passing filtrate without a flocculation step through a second granular medium filter within a second filtertankto a second underdrain cavity to produce a secondary filtrate containing less than 10 percent of the suspended solids present in said untreated wastewater; d. intermittently passing air upwardly through the granular medium of each filter to reduce resistance to flow through the medium and inhibit anaerobic biological activity therein, without disturbing the filter medium integrity; e. intermittently backwashing said first filter with a portion of said primary filtrate and a surfactant and/or oxidizing agent to remove accumulated deleterious materials; and f. intermittently backwashing said second filter with a portion of said secondary filtrate and a surfactant and/or oxidizing agent to remove accumulated deleterious materials.

7. A wastewater treatment process according to claim 6, wherein said secondary filtrate contains less than 20 mg/l suspended solids and 20 mg/l BOD5.

8. A wastewater treatment process for the removal of suspended and/or colloidal materials, including possibly greases and oils, and reduced forms of nitrogen from previously untreated was tewater which contains domestic sewage, comprising: a. passing said wastewater containing domestic sewage to a clarifier or screening device to remove a portion of the materials and clarify said wastewater, without essential addition of chemical; b. passing clarified wastewaterwithout essential addition of chemical through a first granular medium filter within a first filter tank to a first underdrain cavity to produce a primary filtrate; c. passing said primary filtrate through a fixed media filter having nitrifying microorganisms growing on said fixed media, to oxidize said nitrogen in primary filtrate to nitrate and nitrite;; d. passing nitrified primaryfiltratethrough a second granular medium filter within a second filter tank to a second underdrain cavity to produce secondary filtrate containing less than 20 mg/l suspended solids and 20 mg/I BOD8; e. intermittently passing air upwardly through each granular medium to reduce resistance to flow through the media, and inhibit anaerobic biological activity therein; f. intermittently backwashing said first filter with a portion of said primary filtrate and a surfactant and/or oxidizing agent to remove accumulated deleterious material; and g. intermittently backwashing said second filter with a portion of said secondary filtrate and a surfactant and/or oxidizing agent to remove accumulated deleterious material.

9. Awastewatertreatment process according to claim 8, wherein primary filtrate is passed through said fixed media filter at a surface loading rate equal to or exceeding 0.34 GPM per square foot.

10. Awastewatertreatment process according to claim 8 or 9, wherein said primary filtrate is passed through said fixed media filter in a single pass, without recirculation.

11. A wastewater treatment process according to any one of claims 8-10, wherein the temperature of said untreated wastewater is greater than 68OF., and the secondary filtrate contains less than 1.5 mg/l NH4-N.

12. Awastewatertreatment process according to any one of the preceding claims, wherein the filtrate is discharged to a natural water course without further biological treatment other than disinfection.

13. Awastewatertreatment process according to any one of the preceding claims, wherein said deleterious materials removed from said filter or fi Iters, including any carbon particles resulting from attrition of filter medium, are concentrated by gravity thickening and anaerobically digested together with suspended solids removed by said clarifier or screening device to produce fuel gas.

14. Awastewatertreatment process according to claim 13, wherein a coagulant is added to removed deleterious materials prior to gravity thickening.

15. Awastewatertreatment process according to any one of the preceding claims, wherein the oxidizing agent comprises hydrogen peroxide, ozone, and oxygen-containing gas, or a halogenated compound.

16. Awastewatertreatment process according to any one of the preceding claims, wherein the air passing upwardlythrough the granular medium is motivated by filtrate rising within said underdrain cavity, displacing air upwards.

17. A wastewatertreatment process according to any one of the preceding claims, which includes passing said filtrate of step (b) having substantially reduced potential to produce new cellular material to anaerobic biological treatment step wherein biological solids are contacted with said filtrate.

18. A wastewater treatment process according to claim 17, wherein said aerobic biological treatment step comprises passage through a trickling filter without recirculation of trickling filter effluent.

19. A process according to any one of the preceding claims, wherein said surfactant is added to said portion of filtrate with which said filter is backwashed, and said oxidizing agent is injected into a stream of said filtrate just prior to, at, or just following passage of said filtrate into the granular medium.

20. A process according to any one of the preceding claims, wherein said backwashing includes simultaneous spraying of filtrate and/or oxidizing agent on wall surfaces of the filter tank to clean and disinfect said walls.

21. A process according to any one of claims 1-19, wherein the backwashing step includes alternating passage of said filtrate and air to provide high velocity agitation and medium movement at a variable subfluidized rate.

22. A granular medium filtration apparatus for removing suspended and/or colloidal material, including greases and oils, from wastewater, comprising: a first filter vessel having a bottom and side walls; a first filter bed of particulate material adjacent said bottom of first filter vessel and having an upper surface; wastewater inlet means; an underdrain cavity beneath and generally coextensive with said first filter bed; means for draining from said underdrain cavity filtered wastewater after said filtered wastewater has passed through said first bed; a second filter vessel smaller than said first filter vessel, having a bottom and side walls, located within said first filter vessel, above said first filter bed; a second filter bed of particulate material adjacent said bottom of second filtervessel and having an upper surface; ; a second underdrain cavity beneath and generally coextensive with said second filter bed; means for draining filtered wastewaterfrom said second underdrain cavity after said wastewater has passed through said second bed; filtrate tank means for receiving and storing a portion of filtered wastewater drained from said first and second underdrain cavities; pump means for backwashing both filter beds separately or simultaneously with filtered wastewater from said filtrate tank means, to clean said filter beds and suspend deleterious materials in backwashed filtered wastewater above both said beds; conduit means to drain said backwashed filtered wastewater from above the filter beds to a separate treatment step; conduit means to drain excess unfiltered wastewater from above the filter beds to a wastewater dump chamber prior to starting said backwashing; and pump means to return said excess unfiltered was tewaterfrom said wastewaterdump chamberto said wastewater inlet means to be filtered.

23. Apparatus of claim 22, wherein said wastewater inlet means comprises a wastewater conduit communicating with a flow indicator, a flow indicator outlet, said outlet communicating with pipe means to said first and second filter vessels, flow control valves in said pipe means, and further comprising control means wherein said flow control valves are responsive to flow indicator and direct wastewaterflow in excess of a present rate to said second filter vessel.

24. A underdrain structure for supporting a granular medium filter bed support screen, said structure elevated above an underdrain cavity in a lower portion of a filter chamber, comprising: a network of transversely extending bars horizontally spaced to support said screen, an oxidant chamber formed of an upper plate and a lower plate, generally coextensive with an supporting said network of bars, said plates separated in parallel relationship by a plurality of non-limiting spacers, uniformly spaced sleeves extending vertically through the upper and lower plates for allowing a flow of liquid or gas either upward or downward therethrough, uniformly spaced air distribution chambers having lower open ends and upper ends generally closed by said lower plate, means to inject one or more oxidants comprised of air, oxygen, ozone or other gaseous oxidant or solution of hydrogen peroxide or halogenated compound into said oxidant chamber, and means to pass said oxidant from the oxidant chamber into said upward flow of liquid immediately prior to its passage through said screen and into the filter bed.

25. An underdrain structure according to claim 24, wherein said sleeves have orifices therethrough to allow said oxidants to pass from oxidant chamber through said orifices into said upward flow of liquid.

26. Awastewatertreatment process substantially as herein described with reference to Tables i-lll.

27. A granular medium filtration apparatus for removing suspended and/or colloidal material sub- .

stantially as herein described with reference to the accompanying drawings.

28. An underdrain structure for supporting a granular medium filter bed support screen substantially as herein described with reference to the accompanying drawings.