IES71631B2 - Wastewater treatment system - Google Patents

Description

WASTEWATER TREATMENT SYSTEM CATEGORY OF INVENTION This invention may be categorised as a suspended floe biological wastewater treatment system, comprising processes which are widely used for the treatment of biodegradable wastewaters of municipal, domestic and industrial origin.

BACKGROUND OF THE INVENTION Aerobic biological processes of the suspended floe type, commonly known as activated sludge processes, have been in use worldwide for many years. They typically consist of a process unit, reactor or tank which contains an aqueous flocculent biological suspension, comprising bacteria, protozoa and other microorganisms, which are maintained in suspension by a mixing/aeration system and which are used to effect a biochemical removal of dissolved and particulate organic matter from the wastewater being treated.

While there are many variants of the activated sludge process in use, the essential elements of all these processes are a bioreactor which contains a microbial suspension or mixed liquor, a mixing input which keeps the microbial biomass in suspension and an aeration system which transfers oxygen into solution where it is is consumed by the microbial respiration.

The microbial suspension effectively purifies the wastewater by consuming organic matter which, in part, is converted to microbial cell matter, while the remainder is converted to carbon dioxide, water and other byproducts.

Under favourable environmental conditions, ammonia in the wastewater is converted to nitrate by the actions of the nitrifyinng organisms Nitrosomonas and Nitrobacter. These bacteria are slowgrowing and hence will only develop in process reactors that have a sufficiently long microbial residence time or sludge age to permit their accumulation to a significant concentration. Their growth is also influenced by other environmental factors, notably the level of dissolved oxygen in the mixed liquor and the process operating temperature.

In the absence of oxygen many of the bacterial species found in activated sludge processes have the metabolic capability to use nitrate in place of oxygen in their growth process, resulting in the reduction of nitrate to nitrogen gas. This biological nitrate reduction process is known as sn β 31 denitrification. The environmental conditions required for the denitrification process are an absence of oxygen (or at least a very low dissolved oxygen level), a concentration of nitrate and a source of organic matter to provide the carbon substrate for microbial growth.

Phosphorus is an essential nutrient for microbial growth, the typical microbial cell content of phosphorus being of the order of 1.5 to 2% of cell dry weight. The phosphorus content of municipal wastewaters typically greatly exceeds the quantity required for normal microbial growth in processes such as the activated sludge process. The microbial suspension in the activated sludge process can, however, be environmentally regulated to achieve an enhanced uptake of phosphorus, which in many situations may be desirable since the discharge of phosphorus to certain sensitive , waters may lead to their eutrophication. It is known that enhanced phosphorus removal can be effected by subjecting the microbial biomass to an anaerobic/aerobic cycle. During the anaerobic phase of the cycle there is an uptake of the fermentation products of anaerobic decomposition, such as the fatty acids, and an accompanying release of cell phosphorus into solution. In the subsequent aerobic phase of the cycle, the stored fermentation products are oxidised with a simultaneous enhanced uptake of phosphorus.

For the efficient operation of suspended floe processes it is necessary to operate bioreators at a much higher microbial biomass concentration than is generated during the wastewater process flow-through time. This enhanced operational concentration of mixed liquor suspended solids is usually maintained by recycling the settled microbial biomass from an external sedimentation unit.

The mixed liquor suspended solids concentration is maintained at a constant concentration by wasting the surplus biomass produced by the process. Thus, a conventional activated sludge process includes an aerated bioreactor and an associated settling vessel with a sludge recycle facility.

The processes described in the foregoing paragraphs are known processes that have been used in a variety of reactor configurations, in single units or in combination to treat wastewaters, removing organic matter, nitrogen and phosphorus.

It is the object of the present invention to provide an apparatus and method for the biological removal of organic matter, nitrogen and phosphorus from wastewaters, which combines the said aerobic, anoxic, anaerobic and microbial biomass separation processes in a single tank reactor in a way that optimises use of the tank shape and volume and also provides optimal environmental conditions for each stage.

According to the first aspect of the invention there is provided apparatus of cylindrical shape in which the aforementioned microbial process stages are contained in nested concentric zones, the innermost cylindrical chamber constituting the anaerobic zone, followed in outward radial progression by annular zones constituting anoxic, aerobic and sedimentation zones. The incoming wastewater is delivered to the inner anaerobic zone, where a mechanical mixing system brings it into intimate contact with the microbial biomass that has been recycled from the outer sedimentation chamber. The anaerobic mixed liquor flows radially outward by gravity from the anaerobic chamber, flowing in turn through the anoxic, aerobic and sedimentation chambers. The anaerobic chamber discharges to the anoxic chamber through high level openings in the dividing wall between these two chambers. The mixed liquor in the annular anoxic chamber is kept in suspension by a mechanical mixing system that generates a circumferential flow in the annular chamber. The anoxic chamber is in liquid communication with its neighbouring aerobic chamber at both floor and surface levels. The mixed liquor in the annular aerobic chamber is aerated and mixed by an aeration system of the diffused air type. The airlift effect of the rising air bubbles causes a recirculation between the aerobic and anoxic chambers, the recirculating flow being in the outward radial direction at floor level and in the opposite direction at liquid surface level. This recirculating flow transports nitrate from the aerobic chamber, where it is being produced, to the anoxic chamber where it is reduced to nitrogen gas. The mixed liquor from the aerobic chamber is discharged radially outwards by gravity into the outer annular sedimentation chamber through submerged openings in the dividing wall between these two chambers. The clarified effluent is discharged via a peripheral decanting channel at the liquid surface level. The microbial biomass settles to the bottom of the annular sedimentation zone, whence it is recycled by a pump/pipe system carried on a rotating bridge. The rotating bridge is pivoted at the centre of the tank and propelled by a geared motor drive unit travelling on top of the external wall of the tank. Provision is made for the regulation of microbial residence time or sludge age by wasting sludge from the aerobic chamber.

According to the second aspect of the invention the nesting of the process chambers in a concentric fashion optimises the utilisation of the overall tank volume, eliminates the need for transfer pipework between processes and also the need for individual structural water-tight containments for each process.

According to the third aspect of the invention, the said tank configuration makes it readily feasible to provide optimal conditions for the said microbial processes. The simultaneous input of recycled sludge and raw wastewater to the central anaerobic chamber ensures an intimate mixing of these two streams, which are further maintained in intimate contact by the mechanical mixing system in . the anaerobic chamber. The anoxic chamber microbial biomass is kept in suspension by a propellertype mixer with tangential horizontal discharge, which generates a tangential horizontal flow in the annular anoxic chamber. Recirculation between the aerobic and anoxic chambers is essential to transport nitrate from the aerobic chamber into the anoxic chamber where it is reduced to nitrogen gas. The hereinbefore described arrangement, whereby the airlift effect of the aeration system is used to bring about this recirculation, achieves a net saving in recirculation pumping costs since it makes use of the aeration system for recirculation as well as its normal functions of oxygen transfer and mixing. The location of the sedimentation zone in the outer annulus of the tank offers three process advantages over conventional sedimentation tanks (a) it allows the inflow to the sedimentation zone to be distributed over the length of the inner perimeter wall of the sedimentation chamber, thus ensuring a low inflow velocity and quiescent hydraulic conditions favourable to particle settling in the sedimentation zone, (b) the relatively narrow annular width of the sedimentation chamber promotes flow stability and the creation of a favourable hydraulic environment for sedimentation and (c) the relatively narrow width of the sedimentation chamber concentrates the settled microbial sludge in a confined space convenient for its efficient removal by the said rotating pump system, without the need to create special hoppers for sludge concentration as required in conventional sedimentation tanks.

Preferably, the dissolved oxygen concentration in the aerobic chamber is automatically regulated within the band 1-2 mg/1.

Preferably, the mixed liquor suspended solids concentration is maintained at a fixed concentration, the magnitude of which is dictated by the mean microbial cell residence time required for nitrification. This latter depends on the wastewater composition and the process temperature. For a typical domestic wastewater the preferred microbial residence time in the aerobic (nitrification) chamber at an operating temperature of 10 °C is in the range 10 to 12 days.

Preferably the process chambers are arranged concentrically in a single tank, as hereinbefore described. The relative volumes of the individual process chambers are determined by the wastewater composition and strength. In particular, the surface area of the sedimentation zone is determined by the maximum wastewater inflow rate. For example, the preferred surface loading rate in the sedimentation zone should not exceed 1 m3 per m2 per hour.

PREFERRED EMBODIMENT The invention will be understood from the following description of an embodiment thereof given . by way of example only with reference to the accompanying schematic representations shown on Figs 1 and 2. The wastewater inflow (17) is delivered to the innermost anaerobic stage (1), whence it follows a radial outward path (15) through, in turn, the anoxic stage (2), the aerobic stage (3), and the sedimentation zone (4), from which the settled effluent (18) is discharged. The separated microbial biomass is recyled by pumping (6) from the sedimentation zone (4) to the anaerobic zone (1). The pump system (6) is mounted on a rotating bridge (5) driven by a geared motor (14) travelling on the peripheral tank wall (10). The mixed liquor in the anaerobic zone (1) is kept in suspension by a mechanical mixing system (7). The mixed liquor in the anoxic zone (2) is kept in suspension by a mechanical mixing system (8). The aerobic zone (3) is fitted with a diffused air aeration system which transfers oxygen into solution and also keeps the mixed liquor in suspension. The circular dividing wall (12) between the anoxic zone (2) and the aerobic zone (3) is curtailed in height to allow liquid communication between these two zones at floor level and also at liquid surface level. In these circumstances, the airlift effect of the aeration system (9) causes a recirculation (16) of mixed liquor between these two zones, the outward transfer from the anoxic zone (2) to the aerobic zone (3) taking place at floor level and the reverse transfer taking place at the liquid surface level. Provision can optionally be made for enhancing the rate of this interchange by pumping.

Claims (5)

1. Apparatus for the biological removal of organic matter, nitrogen and phosphorus from wastewaters as hereinbefore defined, said apparatus consisting of a cylindrical tank containing a set of nested concentric chambers comprising an innermost anaerobic chamber (1), enclosed by a mechanically mixed annular anoxic chamber (2), in turn enclosed by an annular aerobic chamber (3) having a diffused air aeration system, in turn enclosed by an annular outer sedimentation chamber (4), all chambers being in liquid communication (15), the influent wastewater (17) being directed to the anaerobic chamber (1) and flowing radially outwards (15) by gravity, the clarified effluent being discharged from the perimeter decanting channel (18) in the sedimentation tank (4), the settled sludge biomass being recycled to the anaerobic chamber (1) by pump (6).

2. Apparatus as claimed in Claim 1 wherein the cylindrical tank is partitioned internally to define a set of four nested chambers, comprising an anaerobic chamber (1), an anoxic chamber (2), an aerobic chamber (3) and a sedimentation chamber (4).

3. Apparatus as claimed in Claim 2 in which there is provision for radial outward flow through high level submerged openings in the anaerobic chamber outer wall (13) and in the aerobic outer wall (11), and for recirculation between the anoxic chamber (2) and the aerobic chamber (3) through floor-level and liquid surface level openings in boundary wall (12).

4. Apparatus as claimed in Claim 3 in which the aerobic zone (3) is aerated and mixed by a 5 diffused air aeration system, which also causes a recirculation between the aerobic zone (3) and the anoxic zone (2).

5. Apparatus as claimed in Claim 4 in which the settled sludge in the sedimentation chamber (4) is recycled to the anaerobic chamber (1) by pump system (6) carried on the rotating bridge (5).