SBR plant with chemical and biological nutrient removal

The wastewater has the normal characteristics of civil sewage with a proper ratio from BOD and nutrients.
It requires a good reduction of COD and BOD5 and a high efficiency of removal of nutrients such as nitrogen (N) and phosphorus (P) due to discharge into sensitive areas of eutrophication.
The choice falls on a sequential batch reactor treatment plant (SBR). This type of suspended activated sludge reactor has a high degree of flexibility in handling; In fact, it is possible to set and later modify the timing of operating cycles depending on the actual needs that arise in the time.
The main management problem related to SBR plants is easily solved thanks to a programmable logic controller said more simply (Programmable Logic Controller) PLC.
Its remarkable versatility allows to modify the number and the subdivision of operating cycles thanks to the connection with a person’s computer.
AZU Water technical staff has an extensive experience in PLC programming for sewage treatment plants and is available for customers treatment plant management. For more information, see the SERVICE sections.


The plant is equipped with an pre-treatment tank for the removal of coarse solids, a part of sedimentable solids and fat.

Fats float to the surface and are removed with periodic cleaning operations.

As pretreatment, a septic tanks can be used like SEPTIC ORANGE made by polyethylene with specific ribs to install it even in the presence of groundwater.

Downstream pretreatment there is an equalization compartment. The accumulation is necessary to have only one SBR reactor, while in case of multiple lines could be avoided.
Equalization compartment has the advantage of eliminating peaks flow, perfectly determining the intake of biological load through PLC and equalizing incoming pollutant concentrations.

Removal of organic pollutants

The plant is primarily sized for BOD removal maintaining a low factor of organic load to allow nitrification of aerated mixture and production of stable sludge.
A small plant would be severely penalized by production of poorly stable sludge as it would require frequent disposal, in addition for small dimensions (micro plants) it is unthinkable to add a sludge line.

In technical literature, the typical organic loading values ​​can be found to achieve a complete nitrification and removal of the BOD.

The volume can be determined depending on the project concentration mixture. Once the sludge concentration is fixed, with a suitable safety factor it’s possible to determine the required volume after emptying.
The concentrations used are generally quite cautious to easily deal with cases of low sedimentation sludge.

Nitrogen removal

The assessment of the complete nitrification passes through evaluation of heterotrophic biomass and therefore the quantity of nitrifying bacteria calculated by the ratio BOD / TKN.
The parameter key is the nitrification speed that determines whether the loop subdivision project is correct.

The denitrification takes place in absence of oxygen, so during the loading phases mixed and not ventilated.

Part of the nitrogen is removed biologically and is part of the nitrification/denitrification process; in case of an excess of nitrogen in the effluent you can operate, through a metering pump, the dosage of a bacterial additive to increase the ammonia removal. The bacteria can directly consume ammonia as a nitrogen source, use nitrites and nitrates as a source of oxygen and nitrogen.

Phosphor removal

Phosphorus is partially removed by biological synthesis but in relation to the BOD with very low percentages (100: 1 between removed BOD and P).
For best results, it is necessary to perform a deep phosphor removal. But the Biological Removal, also called BNR (Biological Nutrient Removal), is not recommended in small plants.

The removal of phosphorus is carried out chemically with reagents such as aluminum polychloride injeted by dosing pump.

Among various solutions, in this case, AZU Water choose the post precipitation with a dedicated contact and settling tank downstream the SBR reactor.
Pre-precipitation would increase the reagent consumption since not only phosphorus is removed but also BOD and this will produce a significant amounts of chemical sludge and as a result also low biodegradable and would risk subtracting nutrients to subsequent biological reactions.
Co-precipitation is not advisable for the same reasons and because in low-load organic treatment plants the high oxygen presence and high retention times could cause the solubilization of phophosphorus again.

Therefore, a post-precipitation compartment equipped with baffles with in-line dosage in order to increase sedimentation of chemical sludge is considered the most performing and economically most advantageous solution.

AZU Water makes tailor-made solutions tailored to customer needs.
AZU Water can provide ribbed polyethylene tanks specific even for installation in presence ground water.

For special customer needs, it is possible to adapt and revamp existing installations as long as these are in good condition and do not compromise the functioning of the plant.