Patent Application
20130092612
This invention relates to sewage treatment systems, and particularly to enhancements to a system that includes anaerobic digestion in solids handling.
Anaerobic digestion is included in some sewage treatment plants for reasons of energy efficiency. The anaerobic digestion produces biogas, especially methane gas, which can be used as an energy source in the plant. Anaerobic digestion utilizes different bacteria from aerobic digestion in an essentially oxygen-free environment. The sludge or biosolids resulting from the anaerobic process is high in ammonia. Typically this sludge is dewatered, e.g. to about 20% to 25% and then is disposed of through land application. Filtrate or supernatant from the dewatering of the sludge is high in ammonia, and, in a typical anaerobic plant, is recycled to upstream of the clarifiers on the liquid side of the plant.
On the liquid side of the plant, influent is strained, then typically clarified in a primary clarifier, aerated in an aeration tank where nitrification occurs, and then usually put through secondary clarification. A portion of the solids from secondary clarification known as RAS (recycle activated sludge), which might be at about 1% solids, is recycled to the aeration basin, while the remainder known as WAS (waste activated sludge) typically goes to mechanical thickening and then is delivered to anaerobic digestion, on the solids handling side of the plant, typically at about 3% to 5% solids. This is indicated in the diagram of
The typical system with anaerobic digestion, although treating the high-ammonia filtrate by recycling through primary clarification and aeration (where nitrification occurs), recycles ammonia back into the system, lowering the biological treatment capacity of the aeration tank while increasing energy requirements.
It is a primary purpose of the invention to treat the sidestream of ammonia-laden filtrate from anaerobic digestion, along with waste activated sludge from the main liquid side of the plant following clarification, in a separate subsystem that both thickens WAS and treats ammonia in an efficient way using membrane thickeners, while optionally removing nitrogen in a denitrification stage. This is achieved by the subsystem as described below.
The invention is a physical and biological process that allows wastewater treatment plants, particularly those utilizing anaerobic digestion, to thicken the waste activated sludge (WAS) generated by the activated sludge system while simultaneously removing ammonia present in the filtrate sidestream generated by dewatering equipment.
Pursuant to the invention two influent streams are combined, the filtrate sidestream from the sludge following anaerobic digestion, and WAS resulting after secondary clarification on the liquid side. The subsystem preferably includes a first stage and a second stage. The combined influent stream is introduced to the first stage by entering, in a preferred embodiment, an anoxic basin with the incoming streams providing carbon for heterotrophic bacteria, allowing them to denitrify the sludge within the anoxic basin.
From the anoxic basin, sludge is moved to an aeration basin where nitrifying bacteria present within the WAS are able to convert the incoming ammonia into nitrate in an aerobic environment. Flow from the aeration basin is then pumped into a membrane thickening tank where excess water is removed through the use of membrane separation technology, increasing solids concentration. Sludge in the membrane thickening tank is thoroughly mixed and aerated, allowing further nitrification to take place. A volume of sludge is returned to the anoxic basin where the nitrate produced during aeration, both in the aeration zone and the membrane thickening zone, is converted into nitrogen gas and released to the atmosphere. A portion of the thickened and nitrified flow from the membrane thickening tank is sent to the second stage of the subsystem.
The first stage membrane tank and aeration basin can be reversed in position, with the membrane thickening tank receiving effluent from the anoxic basin, and the thickened sludge from the membrane tank then going to the separate aeration basin. In a further variation, the membrane tank alone can be used for aeration in the first stage of the subsystem, with aeration/nitrification occurring only in the membrane tank. If the anoxic zone is eliminated (as discussed below), then the first tank would be the aeration basin, followed by the membrane separator tank.
The sludge following aeration (either from the membrane or the aeration basin) preferably is divided, with a part being recycled back to the anoxic tank and the remainder, at approximately 2% solids, being delivered to the second stage anoxic basin.
In the second stage anoxic basin the nitrates from the first stage membrane thickening tank/aeration zone are denitrified, along with a recycle flow from within the second stage, converting the nitrates into nitrogen gas released to the atmosphere. Flow from the second stage anoxic basin is delivered to the second stage membrane thickening tank, where sludge is further concentrated by removing excess water using the membranes, in an aerated tank. Like the first stage, the second stage can have a separate aerobic tank either preceding or succeeding the membrane tank. In these aeration zones the remaining ammonia in the sludge is converted into nitrate. The aerated and nitrified sludge is partly recycled back to the anoxic zone and the remainder sent to anaerobic digestion, in a solids handling system such as in
Flows of permeate from the first stage membrane thickener tank and from the second stage membrane thickener tank will preferably be sent back to the head of the plant, e.g. just downstream of “screening” in the example system shown in
In some cases a permeate stream sent to the head of plant will advantageously carry nitrates, since the oxygen in the nitrates can reduce odor from hydrogen sulfide present in the influent to the plant. If this is desired, the anoxic zones shown in the first and second stages can be eliminated from the subsystem of the invention, and in that case the aeration zone will be upstream of the membrane zone.
It is thus among the objects of the invention to treat effluent from a main plant liquid side and from anaerobic digestion in a subsystem preferably of two stages, in which WAS thickening and ammonia treatment are efficiently performed. Preferably (but optionally) denitrification is also achieved in the subsystem. These and other objects, advantages and features of the invention will be apparent from the following description of a preferred embodiment, considered along with the accompanying drawings.
As noted above,
Meanwhile, sludge from primary clarification 14 enters the anaerobic digesters 23. Biogas is emitted as indicated at 24, and this is typically used as an energy source for operating the plant. Anaerobically digested sludge is usually dewatered, as shown at 26, to a solids content which may be about 20% to 25%. The liquid from dewatering, which is high in ammonia and shown at A, typically is recycled to upstream of primary clarification, at 14.
As explained above, pursuant to the invention the flows of WAS at B in
The second stage 34b operates much the same as the first stage 34a. WAS from the stream 46 is denitrified in the anoxic zone 32b, releasing nitrogen gas. The stream then proceeds to a second stage membrane thickening zone 38b, and this may be at about 3% solids. Permeate from the second stage membrane thickening zone flows in a stream 40b to be combined with the stream 40a. This stream, as explained above, is typically sent to the head of the plant, which would be, in the case of the exemplary system of
From the membrane thickening zone the WAS flows to a separate aeration zone 42b in the embodiment illustrated. The effluent from this aeration zone, which, in combination with the aeration occurring in the membrane thickening zone 38b, nitrifies the sludge to treat ammonia and produce nitrate, is divided into two flows: a portion in a stream 40a to anaerobic digestion, which in the case of the exemplary system of
The recycle WAS streams 36a and 36b are indicated as “4Q” in the drawing. This is one preferred embodiment of the system, wherein the rate of flow in the streams 36a and 36b are at about four times the input to the respective subsystems.
As explained above, the membrane thickening zones 38 (a or b) and the aeration zones 42 (a or b) can each be combined into a single membrane thickening/aeration zone if desired. In another alternative, the positions of the aeration zones and the membrane thickening zones can be reversed.
Also as explained above, in some cases it is desired to reduce odor at the head of plant, by neutralizing hydrogen sulfide. In that case the permeate delivered to the head of plant as indicated in the drawing should carry nitrates to break down the hydrogen sulfide and reduce odor. Thus, the anoxic zones 32a and 32b can be eliminated. In that case the membrane thickening zone 38 should not be the first zone in each subsystem; the separate aeration zone would be the first tank of each subsystem.
The above described preferred embodiments are intended to illustrate the principles of the invention, but not to limit its scope. Other embodiments and variations to these preferred embodiments will be apparent to those skilled in the art and may be made without departing from the spirit and scope of the invention as defined in the following claims.
