Single reactor activated sludge system
Graeme Anderson, Business Development Manager for Environmental Solutions International, describes the BNR activated sludge system, recently installed in Queensland's South Caboolture WwTP.
All processes in the BNR system at South Caboolture are carried out in a single reactor
The plant is designed to treat wastewater from a 40,000 p.e and produce an effluent with nutrient levels of TKN 5 mg/l, NH3-N 1 mg/l and total P 1mg/l. ESI's BNR wastewater technology is a two basin arrangement which contains the following processes: carbonaceous treatment; nitrification; denitrification; phosphorus release; phosphorus uptake and biomass selection.
All these processes are carried out in a single reactor. The anaerobic, anoxic, aerobic and clarification zones are created in this single basin by controlled adjustment to flow and aeration.
The ESI BNR process is a variable volume, activated sludge system which operates as a sequencing batch reactor. A typical ESI BNR plant consists of two process basins with an influent reaction zone and air diffuser system to provide aeration and mixing of the biomass. Rotating out-of-basin surface skimmers fitted with positive floating scum barriers completely replace conventional clarifiers. Recycle and sludge wasting are provided by use of simple in-lagoon submersible pumps. A process control centre controls the air-on/air-off sequences, settling time, effluent decanting and flow distribution.
The process design is for a short sludge age and further stabilisation of sludge can be undertaken by using a separate aerobic digester. This sludge digestion basin is intermittently aerated and, with decanting, the settled sludge can be concentrated up to 2-3.5%.
This small reactor zone ensures the biomass is alternately exposed to a feed-starve growth environment. The reactor zone is sized on the basis of floc-load to ensure rapid removal of readily degradable soluble substrate to achieve a predominantly floc-forming biomass.
Biomass is introduced to the reactor zone from the main aeration zone. The ESI BioSelector ensures that the biomass is exposed to feed-starve selectivity pressures at all times. This simplifies the operation of a batch reactor while permitting full reaction sequences in four hour cycles of operation. This also permits bio-phosphorus removal mechanisms to take place, which together with the high level of denitrification affords a substantial saving in energy requirements.
The SBR process is an effective system for the removal of nitrogen. With adequate aeration and sludge age, nitrification will be complete at the temperatures encountered in Caboolture.
Denitrification is achieved in the same basin during the settling and skimming periods. This is achieved by introducing the raw sewage feed to the BioSelector and settled sludge in the basins as the raw sewage continues to be fed to each basin after aeration is completed and up to the time the surface skimming cycle starts. Once this cycle is completed, wastewater is again introduced to this aeration basin.
The raw sewage waste feed ensures the carbon source for the denitrification is present and rapidly depletes the residual oxygen in the sludge blanket. This then ensures that the anoxic zone is created for maximum denitrification.
The SBR plant performs nitrogen removal through simultaneous nitrification and denitrification during aeration sequences.
Maintenance of a high metabolic activity level in a plant with variable reduced loading throughout the year is achieved by maintaining a high 'aerobic' F:M relative to actual loading received. This is achieved by adjusting the air-on and air-off periods and cycle times to reflect the change in reduced loadings to the plant.
A high ammonia nitrogen concentration at the start aeration sequence (27 mg/l) is reflective of a very high influent ammonia nitrogen concentration, taking into account the dilution that results from mixing the bottom water level volume. Over the aeration period, the ammonia levels reduce to effectively zero with the conversion of ammonia to nitrate. The DO concentration is typically < 1 mg/l for most of the aeration time. During the period of low DO concentration the nitrate levels are shown to be at or less than 1 mg/l. Without excess DO, the nitrate generated is simultaneously reduced to nitrogen gas. This simultaneous reduction is evidenced by the low concentrations of nitrate throughout the aeration period.
At the end of the aeration period a high DO concentration is set in order to ensure a time at elevated DO concentration required for the phosphorus uptake process to occur and to prevent the growth of low dissolved oxygen filamentous micro-organisms.
Wastewater treatment when engineered correctly, has a capacity to remove phosphorus using natural biological processes.
The process installed at Caboolture is designed to maximise biological phosphorus removal. This is achieved by the following features:-
Provision of an unaerated initial reaction zone at the head of the plant. The ability of a sequential batch reactor to provide positive aerobic and anaerobic zones.
Provision of a reactive environment which promotes the growth of phosphorus accumulating bacteria.
Recycle of sludge to the unaerated initial reaction zone.
The initial reaction zone at the head of the plant is an essential element in the biological phosphorus removal process. Phosphorus removal will occur because of the natural pre-selection of phosphorus uptake bacteria with the ESI BioSelector.
The aeration basin exhibits an average sludge age of 15 days. This provides a stable sludge which is further stabilised with an additional 11 days detention in the sludge digestion basin. With settling and decanting of the sludge its concentration can be further increased to approximately 2-3%. Following pre-conditioning with polymer the sludge is dewatered to a manageable cake prior to off-site disposal.
Instrumentation and Control
The ESI process has the physical advantage that all of the processes necessary to remove C,N & P are performed in one reactor. The major control strategy is to change the timing of the three various cycles: fill aerate, fill settle and surface skim.Within a normal four hour cycle, there is capacity to handle wide variations in flow and load without operator input.