Basic Biological Nutrient Removal
By: Steven Lambert | Jun 30, 2020
In recent history emphasis has evolved from secondary treatment with the goal on nitrification, which reduces the Ammonia Nitrogen (NH3), to goals to reduce Total Nitrogen and Total Phosphorus discharges to sensitive waters. Conventional secondary treatment is not adequate to reduce Total Nitrogen and Phosphorus to acceptable levels for sensitive receiving waters. The ultimate goal is to control the growth of algae and aquatic plants that rob oxygen from the water.
The Operator requires training and development of extended skill and understanding of Biological Nutrient Removal (BNR). The usual nitrification process now becomes the beginning, not the end, of the treatment objective. Combining phosphorus removal requires the addition of treatment methods in treatment that may be new for some operators. Primary examples are the addition of chemical treatment and/or an anaerobic primary basin.
Reducing the total nitrogen from wastewater is a rather straight forward process that utilizes a controlled version of the Nitrogen Cycle in nature. Nitrification, basic secondary treatment, reduces ammonium, which will convert to Nitrites (NO2) and then to Nitrates (NO3). Both contain oxygen (the O2 and the O3).
The basic aeration basin discharges via the nitrate return pipe or channel to the anoxic basin. The anoxic basin contains little or no oxygen, which stresses the microbes and stimulates a search for oxygen. Any oxygen in the molecular structures will be targeted by the organisms desperate for it. The oxygen atoms are consumed by the biology, which in effect “splits” the molecule. As the Nitrates contain oxygen, the Nitrogen is liberated as a gas and is released into the atmosphere as Nitrogen gas.
The devil is in the details, however. There must be ample food for the organisms to act. A rule of thumb is that three units of biochemical oxygen demand (BOD) to each unit of total Kjeldah nitrogen (TKN), by weight, is required for successful treatment. Supplemental sources of BOD must be added if this is not available in the process already. Online testing of potential supplements usually determine which one to choose.
Also, it should be noted that the Return Activated Sludge (RAS) will be sent to the anoxic basin instead of the conventional aerobic basin.
Phosphorus removal has issues because it must be contained. As a solid, it will not boil off as a gas.
Chemical treatment is an option. Installing chemical treatment equipment is less expensive than construction, and it is easier for the Operator to understand, but it will be expensive over time. The chemical must be constantly replenished. This can also be a stop-gap measure used during construction of a biological system.
The chemical used will most likely be an aluminum or iron salt. It will be introduced at the secondary clarifier inlet or at the primary clarifier inlet. The Phosphorus is joined with the chemical and is retained as a precipitate, or floc. A diligent Waste Activated Sludge (WAS) and disposal program will be required, avoiding washing phosphorus into receiving waters.
Enhanced Biological Phosphorus Removal (EBPR) requires placing an anaerobic primary basin ahead of the aerobic basin. RAS is directed to the primary (anaerobic) basin and WAS to a separate digester. The aerobic basin discharges to the clarifier as usual and the clarifier to receiving waters.
Phosphorus Accumulating Organisms (PAO’s) are grown in the aerobic basin and return as RAS to the anaerobic tank. PAO’s do not multiply in anaerobic conditions, but will “eat” the phosphorus and release it in the anaerobic tank to collect more food from the anaerobic liquor, preparing the organisms to capture more phosphorus and reproduce in the aerobic basin. This constant feeding in the anaerobic and subsequent biological accumulation prevents release to the receiving stream.
Regular WAS to the digester is of prime importance. You cannot hold it forever. A regular and uninterrupted removal of digested sludge to land application avoids overloading and releasing Phosphorus to receiving waters.
Both Nitrogen and Phosphorus removal will most likely be required in many situations. In this case the normal Nitrogen removal basin layout remains the same but with the addition of an influent anaerobic basin. RAS is directed to the anaerobic basin as in the Phosphorus removal layout. There is also the nitrate return from the Aerobic basin to the anoxic basin. This can insure Phosphorus removal by limiting the possibility of stray oxygen from invading the anaerobic basin.