| Custom designs of Conventional Systems |
These plants are designed to treat wastewater from all domestic and some industrial sources. The plant is comprised of two circular tanks (other shapes are available depending on site conditions), an aeration tank and settling tank. In case a denitrification step is needed, an extra tank (anoxic tank) will have to be added prior to the aeration tank. Tanks can be either made of reinforced concrete, GRP, mild steel or SS304 depending on site and budget constrains.
The biological wastewater treatment system is based on the extended aeration activated sludge process. After passing though a screening process (either manual or automatic screen), wastewater flows by gravity or is pumped to the aeration tank. This tank contains ‘activated sludge' or ‘mixed liquor' which is a mixture of wastewater undergoing treatment and micro-organisms (in flocs) that facilitate the removal of organic material from the wastewater. The mixed liquor undergoes periodic aeration to ensure that the contents of the tank remain uniformly mixed and to ensure provision of sufficient oxygen for biodegradation of organic material by the microorganisms.
Mixed liquor flows over a weir, through the pipe work and into the settlement tank. The microorganisms that are agglomerated in flocs, settle to the bottom of the tank and the clear, treated water is discharged from the settlement tank by gravity. The settled flocs (sludge) flow by gravity to a Return Activated Sludge (RAS) pumping station. From here the sludge is returned back to the aeration or wasted to the sludge holding tank.
These plants are designed to produce effluent to the standards of 20 mg/L BOD5 (Biodegradable Oxygen Demand) and 30 mg/L SS (Suspended Solids).
The plant may be modified to facilitate nutrient (nitrogen (N) and phosphorous (P)) removal. Nitrogen removal (nitrification /denitrification) is dependent on the ambient plant operating temperature (>8oC) and the aeration regime used. Phosphorous removal is facilitated by the addition of chemicals to the system. Additional sludge is produced by the system in the course of phosphate removal.
Effluent quality may be further improved by additional clarification (10 mg/L BOD5, 10 mg/L SS) or disinfection (usually with chlorine) to provide water suitable for irrigation or re-use.
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