BNR bioreactors and secondary clarifiers are the core facilities for biological treatment of organic pollutants and nutrient pollutants such as ammonia and phosphorus. (Ammonia is the primary contaminant. Nitrogen – a gas – is produced from the pollutant ammonia.) These pollutants are removed through the metabolism of a variety of microorganisms.
The bioreactor is divided into several cells. In some cells, air/oxygen is introduced through fine bubble diffusers at the tank bottom for the use of aerobic microorganisms in removing pollutants such as ammonia; while in other cells, no air is required for anoxic or anaerobic microorganisms to remove nitrogen and phosphorus.
Through the combined efforts of different microorganisms (also called activated sludge), those pollutants are removed from wastewater. The pollutants serve as the food source to maintain the metabolism of the microorganisms and to generate new microorganisms. Ammonia is converted to nitrate and then to nitrogen gas that is released into the atmosphere. Phosphorus is accumulated in the cells of the microorganisms and becomes part of the plant biosolids (sludge).
Depending on the compositions of pollutants in wastewater, a carbon source (e.g. methanol) may be added to help microorganisms in their metabolism of the pollutants. VFAs required by some microorganisms in the removal of phosphorus are supplied by the fermenters.
The mixed liquid of activated sludge and water from the bioreactors flows to four large circular clarifiers where sludge settles to the bottom. Settled activated sludge is then pumped back to the bioreactor to maintain the mass of microorganisms. Excess activated sludge is pumped to the dissolved air flotation facility for thickening. The clarified effluent, with over 95 per cent of the organic pollutants removed, flows to the ultraviolet facility for disinfection. Scum formed on the top of the secondary clarifiers is sent to digesters for treatment.
What is BNR?
BNR stands for Biological Nutrient Removal (BNR). This process uses microorganisms to remove the elements nitrogen (N) and phosphorus (P) from wastewater. Nitrogen and phosphorus are essential to the growth of plants and therefore are known as nutrients.
What are the forms of N and P in wastewater?
In untreated wastewater, nitrogen comes in the forms of organic nitrogen and ammonia; phosphorus is in the form of orthophosphate, polyphosphate and organic phosphate.
Origins of N and P in wastewater:
Domestic wastewater typically contains approximately 35 ppm (parts per million) ammonia and 7 ppm phosphorus. Some industries may produce wastewater with high nitrogen and phosphorus concentrations.
Reasons to remove N and P:
Treated effluent with a high concentration of nitrogen and phosphorus can be detrimental to aquatic life as well as the function of waterbodies.
Nitrogen in the form of ammonia is toxic to fish. The discharge of nitrogen and phosphorus will stimulate the growth of algae and aquatic plants in lakes and reservoirs and deplete dissolved oxygen in waters, which is also toxic for aquatic life.
The presence of algae and aquatic plants may also appear unsightly, interfering with uses of waterbodies for recreation, water supplies and fish propagation, etc.
The sketch shows the layout of one of our 3 bioreactors. From it, we can see that the primary effluent goes into the RAS-denitrification cell and anaerobic cell). RAS is back to the RAS denitrification cell and nitrate in RAS gets denitrified there. In the anaerobic cell, phosphorus release takes place with the assimulation of VFAs by microorganisms. Nitrified mixed liquor is pumped back to the Anoxic cells and nitrate in it gets denitrified. In aerobic cells, removal of organic matters, nitrification, and uptake of phosphorus take place.
Our effluent ammonia limit is 5 mg/L for summer (Jun to Nov) and 10 mg/L for winter (Dec to May). Effluent total phosphorus (TP) limit is 1 mg/L. Achieving removal efficiency as high as possible is a principle, but we need to optimize the process for the removal of ammonia and phosphorus under the limited VFA amount and try not use or use less chemicals such as methanol and alum so as to reduce the treatment cost.
If we achieve a very low effluent ammonia (e.g. 1 mg/L), we will get a high effluent nitrate, which will consume more VFAs and leaves no enough VFAs for the phosphorus removal. Then, methanol needs to be added to reduce the nitrate concentration, or alum needs to be added to reduce the effluent TP. Consequently, sludge production increases and so does the treatment cost. Thus, we should try to:
- Reduce the effluent nitrate (NO3) below 10 mg/L or lower; As we know, one part of nitrate brought back to denitrification zone will consume 5 parts of VFAs;
- Achieve effluent ammonia 1 ~ 3 mg/L during summer and 3 ~ 7 mg/L during winter by adjusting air input and nitrification and/or adjusting MLSS;
- Maintain a low DO at the end of aerobic zone as one part of DO brought back to denitrification will consume 2 to 3 parts of VFAs;
- If above three measures still could not reduce the effluent nitrate, methanol should be added to improve denitrification (reducing NO3) or alum added to reduce effluent TP.
Thus, if there are high DO and high nitrate after bioreactors, they (in the RAS) will consume a lot of VFAs, leaving not enough VFAs for phosphorus removal and thus impacting phosphorus removal. DO and nitrate will consume VFAs first and the P removal microorganisms just wait for the remaining VFAs and if the remaining VFAs are not sufficient, the P release action in anaerobic zone and P uptake action in aerobic zone will not take place well, resulting a poor biological P removal and high P concentration in effluent.
If a wastewater treatment plant has enough VFAs (from influent or/and fermenter), it can treat the wastewater to get an effluent ammonia to a very low level (e.g. below 1 mg/L) and also a low total nitrogen or TKN while achieving a good phosphorus removal. However, if the VFAs amount is tight, you need to have a better management of its use.
Then, how about effluent BOD and TSS?
If a wastewater treatment plant effluent can meet the permits for ammonia (e.g. 5 or 10 mg/L) and total phosphorus (e.g. 1.0 mg/L), it will absolutely meet the permits for BOD and TSS. Therefore, do not worry about the effluent BOD and TSS. Reasons: the nitrification will take place only after the organic matter (BOD) is depleted. Thus, if the plant process has a nitrification taking place, the remaining BOD in the effluent will be very low (e.g below 10 mg/L). The solids in effluent contains about 3 to 6% phosphorus. Thus, if effluent TSS is 15 mg/L, the phosphorus from it will be 0.5 to 1 mg/L. Plus the soluable phsophorus, the effluent total phosphorus will likely approch or exceed 1.0 mg/L, depending on the removal of soluable phosphorus by a BNR system. In other words, if effluent total phosphorus is less than 1.0 mg/L, the effluent TSS will be low.
Control of sludge blanket in secondary clarifier:
It is preferred to run a sludge blanket of 2 to 4 ft (0.6 to 1.2 m) in the secondary clarifiers. Under the blanket depth, the DO amount to be carried back through RAS to the bioreactors will be minimized. It is risky to run a blanket depth over 7 ft (2.1 m) as solids carried over to the effluent may take place (depending on the operating MLSS concentration and surface solids loading to the clarifiers), resulting in a high TSS and high TP in effluent. Thus, if a high sludge blanket is observed, RAS rate needs to be raised to pump back the solids stuck in the clarifiers and reduce the blanket depth. At the same time, MLSS may needs to be reduced (depending if it too high) by increasing the WAS rate.
Designed and operating MLSS:
Designed MLSS is based on the assumption of some parameters and assumption of some operating condition, such as use of 2 mg/L of DO in each aerobic cell and a corresponding nitrification rate. However, the actual will be different. Based on our experience, if the secondary clarifier can handle, the operating MLSS can be much higher than the designed value. The advantages of doign so include: 1). well handle the fluctuation of influent TKN loads or spike of TKN load without producing high effluent ammonia concentration; 2). allow use of low DO in the aerobic zones, which reduces the amount DO in the nitrified mixed liquor recycle flow carried back to the anoxic zone; and 3). reduce the volume of WAS wasted.
Adjustment of operating parameters and effluent quality or nitrification change:
Once an operating paramenter is adjusted such as MLSS or DO, in one or two days, you should see the change of effluent ammonia. If there is no change in effluent ammonia concentration, the MLSS or nitrifiers are not sufficient and you can increase the MLSS. If the improvement is not sufficient, you should do more adjustment such as DO and MLSS. Don’t wait for one MCRT (mean cell residence time and the old term is solid retention time or SRT), which will be about 10 days or longer. If you wait for one MCRT to see the result, you just waste time and may get a poor effluent quality for longer days. Somebody may say to wait for one MCRT for the process to be stable so as to make another adjustment, and we can say that this is not a correct way for conducting process startup or adjustment.