WASTEWATER TREATMENT

The biological wastewater treatment process is the most proven technology for organic matter removal and nutrient removal. Based upon the growth type of the microorganisms, the biological wastewater treatment can be divided into two major groups, suspended growth process (e.g., activated sludge) and attached growth process (e.g., trickling filter). 

In a suspended growth process (e.g., activated sludge), the microorganisms are suspended in mixed liquor and have more opportunities to capture food sources (organic matter and nutrients) and dissolved oxygen. The advantages of a suspended growth process include higher quality effluent (90-95% BOD5 removal), nitrogen and phosphorus removal, and the flexibility to adapt to minor pH, organic and temperature changes. However, the major disadvantages of a suspended growth process include sludge bulking, highly skilled labor requirements, and sensitivity to shock loads, metallic and other toxic compounds. 

An attached growth process (e.g., trickling filter) consists of a bed of permeable medium of either rock or plastic media used to host the microorganisms. The organic matter (BOD5) and nutrients in the wastewater diffuse into a film, where it is then metabolized. To acquire a good quality effluent (80-90% BOD5 removal), multiple stages of the attached growth system may be required. 

WSI’s BCR (Bio-Chip Reactor) system is an enhanced activated sludge process with the biomass attached to a small bio-media that is suspended in the mixed liquor. The BCR system combines the advantages of a suspended growth process and an attached growth process. Compared to conventional activated sludge and extended aeration system, the BCR system has the advantages of providing greater biomass concentration, therefore increasing treatment capacity, eliminating sludge bulking concerns, stabilizing process operations, reducing sludge production, enhancing sludge settleability, and lowering costs for operations and maintenance. Compared to other attached growth processes, the BCR system has the advantages of providing a higher quality effluent, is easily adapted for nitrogen and phosphorus removal, provides more opportunities for contact with organic matter nutrients and eliminates odor and insect problems. 

The BCR system is mostly self-regulating and does not require daily measurements of mixed liquor suspended solids (MLSS), sludge volume index, sludge settleability, Return Activated Sludge (RAS) and Waste Activated Sludge (WAS) rate calculations for proper operation. This reduces the skill level required for successful operation and daily time requirements to maintain the system. This is critical for small community wastewater treatment facilities that do not have the resources for full-time management. 

The DAF unit acts to remove the suspended solids (Biomass) from the BCR reactor effluent. The BCR reactors convert soluble nutrients into cell mass and the DAF removes the cell mass and inorganic suspended solids from the reactor effluent through dissolved air floatation. The DAF pump recirculates DAF effluent and flash mixes it with ambient air (aspirated on the suction side of the DAF pump). The air dissolves into the effluent under the pressure of the pump.

The air-water solution is then passed through a saturation tank to allow additional time for the air to dissolve and to remove any coalesced air bubbles. The solution is then expanded across diaphragm control valves leading into WSI’s proprietary diffusers inside the body of the DAF. By reducing the pressure of the solution rapidly (expanding), the dissolved air is forced out of solution in the form of micro-bubbles (90% of the bubbles formed are less than 10 µm in diameter).

The small bubbles collect under the biomass floc and float it to the surface, forming a thickened sludge blanket on the surface of the DAF that is raked directly into the aerobic digester. The DAF provides greater thickening of the sludge as well as a reduced footprint from a conventional clarifier. Settled sludge is periodically pumped to the aerobic digester via a Seepex progressive cavity pump.