CA1099831A

CA1099831A - Wastewater treatment

Info

Publication number: CA1099831A
Application number: CA311,466A
Authority: CA
Inventors: Paul V. Knopp; Walter Burant, Jr.
Original assignee: Sterling Drug Inc
Current assignee: STWB Inc
Priority date: 1977-09-19
Filing date: 1978-09-18
Publication date: 1981-04-21
Also published as: NL7809523A; DE2840764C2; FR2403306A1; ZA785217B; GB2004533B; JPS5456243A; DE2840764A1; GB2004533A; CH633238A5; FR2403306B1; BE870540A

Abstract

ABSTRACT OF THE DISCLOSURE
A process for treating wastewater comprising the following steps:
a. contacting wastewater in a stream in a biological contactor apparatus which has a multiplicity of surfaces therein alternately immersed in the liquid and the gas above the liquid, for a period of time of at least 1/2 hour;
b. adding to the stream an adsorbent capable of adsorbing impurities from the liquid; and c. removing accumulated suspended solids from the contactor apparatus at a rate equivalent to the rate at which solids accumulate within the contactor.

Description

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This invention relates to a method for ~reating wastewaters with an adsorberlt and a rotat-ing biological contactor.
A variety of methods are available for the biological treatmant of sewage and industrial waste-waters. Commonly used methods are the activated Yludg~ proaess using air and pure oxygen and the trickling ilter. Mor~ recently the rotat'ng bio-logical contactor, e~g., as described in U.S. Patent 3,557,954, has found application in which a series of dlscs on a common shaft are rotated in the body o~ the wastewater alternately immersing a biological film formed on the disc in the liquid and exposlng the film to the air above the liquid ko provide oxy-gen for maintaining a biological film on the surfaceof the disc to effect biological oxidati~n of th~
wastewater. Reduced energy consumption f~r oxygen tran fer i9 claimed in such a system which can be designe~ for a variety of applications ranging from ~0 ordinary carbonaceous BOD5 removal to biological nitrification The broad concept of passing wastewater in countercurrent flo~ with continuously moving aativat-ed carbon to remove organic content of the water is disclosed in U.S. Patent 3,763,040.
More recently, it has been suggested that powdered aativated carbon be added to the activated sludge process to effect ~mprovements in performance of ~he activated sludg~ ~ystem as taught in U.S.
Patent 3,904,518. Benefits claimed are improved stability, improved liquid solid~ separati.on, and a - hiyher degree of treatmsnt. The method taught in : .
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U.S. Patent 3,904,518 is a sinyle stage suspended slurry contact system in which the carbon is con-tacted only with the equilibrium concentration of adsorbable organics ln the wastewater in the presence of activated sludgeO For more efflcient contact of the carbon with adsorbable material, it is desirable to effect a c3untercurrent contact with water to expose the carbon incrementally or continuously to a higher concentration of adsorbable material in the 0 wastewater.
Japanese Patent Publicakion No. 139169~76 discloses a method of biotreatment of wastewater comprising contacting the wastewater with activated carbon fixed on the surface of a fixed bed or disc.
As more stringent treatment requirements for wastewater are developed, it has become necessary to consume greater quantities of energy~ For example, where oxidation of reduced nitrogenous material is desired, oxygen requirements are increased markedly requixing greater energy consumption. Further, the longer cell residence tim~s result ln greater con-version of cell mass to C02 and H20 requiring still greater oxygen transfer and energy consumption.
Further, the more stringent treatment requirements may make necessary the additlon of tertiary treatment to remove residual organic materials as well as any toxic organic materials present in the wastewater.
The invention relates ~o a process for treating wastewater comprising the following steps:
(a) contacting wastewater in a stream in a treatment ~one with a multiplicity of surfaces alternately immers~d in the liquid and the gas, usually air, above the liquid for a period of tlme of at least one-half hour;
~b) adding to the contactor an adsorbent capable of adsorbing lmpurities from the liquid; and (c) removing the ~uspended 301ids from the contacting zone at a rate equivalent 33~

to the rate at which solids accumulate within the contactorO
The preferred adsorbent is powdered activa~ed carbon, although the use of granular acti-vated carbon or other adsorbents such as Fuller'searth~ fly ash and the like is contemplated. A pre-~erred range of concentration of adsorbent is from 25 to 5000 milligrams per liter.
The invention i5 now described with reference to the accompanylng drawings, wherein:
Figures l and 2 show the basic process; and Figures 3, 4 and 5 illustrate detailed embodiments to effect countercurrent contact of the biomass and carbon slurry with the liquid phase.
Referring to Figure l, raw wastewater entering at lO receives a preliminary treatment con~
sisting of screening and grit removal in a device 12 for this purpose and passes to an optional primary treatment step consisting normally of plain sedimenta-tion at 14 wh~re suspended floating, and settling solids may be removed as by line 16 and disposed of by conventlonal means 18c The settled wastewater is passed then through a line 20 to a basin 22 in which a series of discs 24 are mounted on a common shaft 25 and rotated by convenient means to lie alternately in the liquid in the basin and in the gas space above the liquid Normally flow through the basin is in a direction parallel to the shaft and perpendicular to the planes of the rotating discs.
In some instances, flow is perpendicular to the shaft and parallel to the rotating discs.
Powdered activated carbon is added to the mixturP in the basin as at 26, e.g., to provide adsorption of biodegradable and non-biodegradable material contained in the wastewater. The rate of carbon addition varies depending upon the trea~ment level required as well as the compo~itlon and char-acteristics of adsorbable material contained in the wastewater. Normally from 10 to 500 mg/l of treated wastewater are required, but or some industrial ' ' :
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wastes, quantities as great as 5000 mg/l of waste-water tr~atment may be required.
~ he film formed on the surface of the disc consists of a mixture of biomass and adsorbent (acti-vated carbon). Surprisingly, it has been obs~rvedthat the film has the unusual property of accumulat-ing the adsorbent into its structure, thereby carry-ing the adsorbent on the disc surface along with the biomassn The ratio of the mass of biological oryanisms to activated carbon will depend essentially upon the solids residence time of the system and the composition of the waste being fed to the system.
It is apparent from the Examples which follow that the adsorbent not only aids in removing adsorbable material from the wastewater, but also has the effect of substantially increasing the residence time of adsorbable and slowly biodegradable substances, and also has the effect of greatly increasing the over-all solids residence time of the system.
This effect is beneficial when nitrifica-tion ~biological oxidation of ammonia nitrogen) is desired since it permits the accumulation of slow growing ni*rifying bacteria.
A further unexpected benefit derived from the addition of activated carbon to the contacting æone of the rotating biological contactor results from the carbon's tendency to reduce foaming. In many applications, the rotational speed of the con-tactor is limited by, am~ng other things, the tendency for foaming to occur in the contactor.
Foaming may occur due to the presence in the waste-water of surface active material or due to surface active exo~enous organic materials produced by the organisms themselves. Since most, if not all of these materials are adsorbed onto the ac-tivated carbon, foaming is reduced or eliminated, permitting higher rotational velocities and concomitant oxyyen transfer.
A still further surprising result of the addition of activated carbon to the rotating bio-, . ' ' '' ~ . .

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--5--logical contactor ls the effect of the carbon addi-tion upon the overall mass transfer rate of oxyg~n.
Measurements o~ the overall oxygen mass transfer rate (KLa) in the contactor is as much as twenty-five per cent better than in the ordinary biologicalcontactor. The improved mass transfer rate has a dual impact upon the performance of the contactor.
First, a higher transfer rate permits slower rota-tional speeds and hence lower power costs; and second, the oxygen profile across the film on the disc is improved providing deeper penetration through the film with a greater resultant active aerobic biological mass. This effect is particular-ly important in nitrifying systems where the nitrify-ing organisms must be exposed to dissolved oxygenconcentrations in excess of .5 mg/l to be ef~ec~ive.
While the carbon can be added at various points within the basin, advantages can be derived by adding the carbon at various points through the longi~udinal dimension of the basin~ For example, as shown in Figure 3, if the carbon is added uniform-ly along the longitudinal ~enath of the basin and the solids are removed as at 28, 28-1, 28-2 and 28-N
along the longitudinal length, carbon contact con sists of essentially a series of si~gle stag~ con-tactors. The rate of addition at each point can be varied to control the loading on the carbon to match the treatment requirements of each successive stagen As is well-known, in adsorption systems, it is desirable to more efficiently load the adsorbent with adsorbate by mGving the adsorbent in counter-current relationship with the adsorbate. In suspend-ed slurry systems, this is usually done by providing a separation step following each contacting stag~.
S~lids sepaxated at ~ach stage are moved upstream, countercurrently to the liquid stream. Figure 4 illustrate~ how this can be achieved in a rotating biological contactor by introducing the adsorbent (activated carbon) at the downstream end and provid-ing a means of moving the carbon upstream, such that : . . : . . .: . . . . , , ~ , , .
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particles separating out at the bottom of the con-tactor tend to move in countercurrent manner along the bottom of the basin providing any number of equivalent countercurrent stages, thus providing maximum adsorption efficiency of the carbon. A
variety of means can be utilized to effect counter-current motion of the solids. For example, the discs can be mounted in a chamber with a sloping bottom such that the suspended parkicles tend to move in an upstream direction as they are disturbed at the bottom of the chamber.
For systems in which flow is introduced perpendicular to the shaft and parallel to the discs, a series of parallel shafts rotated such that the direction of rotation of the disc in ~he liquid is counter to the flow will produce counter-current motion of the æolids in the system.
Alternatively, the periphery of each disc 24 can be fitted with plows or blades 30p directing the net solids flow in a countercurrent direction as illustrated in Figure 5. An optional relation-ship exists between the angle of attack of the plows, the effective rate of countercurrent movement, and power consumption to rotate the disc.
In another embodiment, the disc may be ~esigned so that its periphery takes the shape of a screw with a small angle of slope such that when the disc rotates, the solids are pushed in an up-stream direction. The downstream surface could re main flat or preferably takes the corresponding shape of the upstream face.
Other methods of achieving countercurrent flow may be apparent after study of the above examples. The fu~damental principle involved in the above is that the adsorbent added is moved in a countercurrent relationship with the bulk of the liquid medium thereby effecting higher adsorptive loadings.
In practice, it is never possible to effect complete suspended solids separation within :
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the chamber ln whlch the discs axe rotated because of turbulence created by the rotating disc. To effect improved separation, a quiescent chamber optionally is provided at the downstream end of the contactor as at 32, Figure 2 The quiescent chamber has a steeply sloped bottom 34 to direct the solids upstream into the region where the disc can direct it further upstream. Alternatively, a conventional collector mechanism can be provided to positively convey the solids to the region of the disc for transfer countercurrently upstream.
Similarly, a secondary clarifier 36 can be provided to further clarify the liquid stream.
Material settled in this clarifier as at 38 can be directed to the inlet end of the contacting chamber as by line 39, Figure 1, or it may be mixed with the underflow from the contactor at 42 for further processing, for example, regeneration of the carbon adsorbent or to solids disposal.
Virgin carbon can be added to the system to provide effluent polishing in situations where the spent carbon is regenerated.
Spent adsorbent at 42 along with the associated bicmass can be optionally thickened and disposed of by conventional means 44, or it could be regenerated as at 46 and returned to the contactor for reintroduction into the treatment step at various points in the contactor, see line 48O
The treated wastewater 40 can be further proc~ssed through an optional sand filter 50 and disinfected and dischar~e~ to the receiving stream.
Suspended solids captured in the sand filter can be backwashed and returned upstreàm to any of a number of upstream points in the system shown in Figure 1. Other conigurations are possible. For example, biological nitrification and denitrification can be achieved by operating a first stage under aerobic conditions to achieve oxidation of ammonia and a nitrogen removal stage in anaerobic conditions to achieve denitrification o nitrate and nitrite .

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nitrogen to elemental nitrogen.
The following examples are intended to illustrate the process.

The laboratory rotating biological con-tactor system consisted of fifty-two 10" diameter 1/4" thick polyethylene discs suspended from a horizontal shaft and placed in a hemispherical-shaped 20 liter tank equipped with a cone settler near the outlet end. As the discs rota~ed, approx-imately 40 per cent of the surface area thereof was immersed in liquid. Domestic sewage primary effluent was metered into one end of the tank. The mixed liquid overflowed from the opposite end into a clarifier equipped with a slow moving paddle.
Two recycle lines were used to return thickened settlings from the clarifier and cone settler to the inlet end of the tank at rates of 40 ml/min. and 12 ml/min., respectively, Initially, the system was operated on primary effluent for 19 days in order to build up layers of biomass on the discs. During the next 8 days, data on samples taken continuously were obtain-ed on the system. On the 28th day of operation, 300 grams of activated carbon were added near the tank inlet. Daily, for the following 38 ~ays, 1 liter of mixed liquor was withdrawn from the cone and 15.0 g.
dry activated carbon were added to the tank. The mixed liquor containing biomass and carbon was filter-ed. Dry filter cake weights rangin~ from 0.3 to 26.4~. were obtained. A feed rate of about 100 ml/min.
was maintained throughout the experiment. Rotation speeds o~ 6 and 10 RPM were used to maintain the dissolved oxygen range of 0.5 to 5.0 and 0.6 to 6.0 mg/l in the systems without and with carbon. The average characteri~tics of the streams are listed in the following ~able.
Note that the system operated with carbon showed better COD and BOD5 reduetlon as well as nitri~
fication than the system operated without carbon.

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EXAMPLE I
__ Without Carbon Carbon Added Feed Effluent Feed Effluent ~ _ COD, mg/l 160 72 186 36 5 ~ COD Reduction - 54 - 81 BOD, mg/l 64 16 34 2 % BOD Reduction - 7S - 94 Total Kjeldahl Nitrogen, mg/l 43.3 31.9 50.5 14.4 10 % Total Kjeldahl Nitrogen Reduction - 26 71 Suspended Solids, mg/l 32 24 38 9 Suspended Ash, 15 mg/l 9 3 17 2 Ammonia Nitrogen, mg/l - - 39.8 10.5 Nitrites as Nitro-gen, mg/l - 1 0.1 1.5 Nitrates as Nitro-gen, mg/l - 1 1~5 27.8 - Total Phosphorus, m~/l 9.4 8.712.5 11.5 pH 7O3 7.37.2 6.6 EXAMPLE II
The apparatus described in Example I was employed with a feed rate of about 200 ml/min. for 30 days. No comparative data of the system without carbon were obtained. The values for chemical oxygen deman~, biochemical oxygen demand and Xjeldahl nitro-gen reductions of this system without carbon at the faster feed rate would not exceed the flgures for the 100 ml/min. rate listed in Example I. Daily filt~r cake dry weights varied from 4.08 to 26.14 g. and averaged 11.50 g. per day. The average data of the 200 ml./min. experiment are listed in the follow-ing TableO

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EXAMPLE II
Carbon Added Feed Effluent COD, mg/l 215 49 5 % COD Reduction 77 BOD~, mg/l 60 7 % BOD Reduction - 88 Total Kjeldahl Nitrogen, mg/l 48.9 lZ.7 % Total Kjeldahl Nitrogen Reduction - 74 10 Suspended Solids, mg/l 45 7 Suspended Ash, mg/l 7 2 Ammonia Nitrogen, mg/l 40.2 5.2 Nitrites as Nitrogen, mg/l 0.1 1.1 Nitrates as Nitrogen, mg/l 1 23.2 Total Phosphorus, mg/l 14.6 13.1 pH 6.9 6.9 Note that in spite of the doubling of the loading rate to the system, comparable BOD5 and COD
reductions and nitrification of the ammonia n;trogen were obtained when compared to the results of Example I.
EXAMPLE III
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The apparatus described in Example I was modified to achieve a countercurrent flow of activat-ed carbon and primary sewage effluent. The conesettler was moved to the inlet of the trough. Clarifier and cone r~cycle slurries plus the virgin carbon were introduced near the trough outlet. Spent carbon slurry was withdrawn daily from the cone settler. Average feed rates of 111 ml. primary sewage effluent per minute and 94 mg activated carbon per liter of efflu~
ent were used. An average of 16.1 g. dry solids were removed daily. The average analytical data are pre-sented in the following Table.

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EXAMPLE III
Feed Effluent COD, mgjl 227 43 % COD Reduction - 81 5 BOD5, mg/l 60 7 ~ BOD Reduction - 88 Total Kjeldahl Nitrogen 48.9 12.7 % Total K~eldahl Reduction - 74 Suspended Solids, mg/l28 10 Suspended Ash, mg/l 2 Ammonia Nitrogen, mg/l47.411.8 Nitrites as Nitrogen, mg/l 0.1 0.5 Nitrates as Nitrogen, mg~l 1.5 17.0 Total Phosphorus, mg/l16.014.7 15 pH 6.5 6.3 EXAMPLE IV
To illustrate the effect of the addition of activate~ carbon to the contactor upon the over-all mass transfer rate of oxygen, tesks were con-ducted in accordance with the methods described byEckenfelder and Ford* to determine Alpha, the ratio of the mass transfer rate of oxygen in ~he carbon-biomass slurry and biomass aloneO Tabulated below are the Alpha values observed:
K a Wastewater 25 Alpha (~) = ~ ~
Biological Biological Plus Wastewater SysternActivated Carbon .
Pharmaceutical Waste - 2.43 Domestic Waste 0.821.27 Municipal/Industrial Waste 0.811.05 The carbon addition has the effect of greatly increasing the overall oxygen mass transfer rate (KLa) when compared to the biological system and permits equivalent oxygen transfer at greatly reduced peripheral speeds.
* "Experimental Procedures for Process Design"
Water Pollution Control, Pemberton Press, Jenk~ Publishing Co., Austln, Tex. (1970), pp. 103-112.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for treating wastewater compris-ing the following steps:
a. contacting wastewater in a stream in a biological contactor apparatus which has a multiplicity of surfaces therein alternately immersed in the liquid and the gas above the liquid, for a period of time of at least 1/2 hour;
b. adding to the stream an adsorbent capable of adsorbing impurities from the liquid; and c. removing accumulated suspended solids from the contactor apparatus at a rate equivalent to the rate at which solids accumulate within the contactor.

2. A process according to claim 1, in which the adsorbent is powdered activated carbon.

3. A process according to claim 1, in which the adsorbent is added at the downstream end of a biological contactor apparatus and is moved by the surfaces in the contactor apparatus in counter-current relationship with the bulk of the liquid flow.

4. A process according to any one of claims 1, 2 and 3, in which the amount of adsorbent added ranges from 25 to 5000 mg/1.

5. A process according to any one of claims 1, 2 and 3, which includes removing solids from the wastewater prior to the contacting step.

6. A process according to any one of claims 1, 2 and 3, in which the contacting surfaces com-prise a series of discs mounted on a common shaft which is rotated to alternately immerse the disc and its associated biological film in the liquid and in the gas above the liquid.