Yimei's Sequencing Batch Reactors (or Sequential Batch Reactors, SBRs) are a unique secondary aerobic biological treatment process ideally suited for several industrial applications. Typically SBRs reduce the organic and nutrient loadings in medium strength wastewater to comply with surface water discharge limits or as a pre-treatment stage to tertiary recycling. While there are several configurations of SBRs, the basic process is simple. An installation comprises one or more tanks equipped with a common inlet allowing feedwater to be directed to either tank. There are four stages to SBR treatment: fill, aeration, settling and decanting.
The aeration stage adds air to the mixed solids and liquid (mixed liquor) either via fixed or floating mechanical pumps or by blowing it into finely perforated membranes fixed to the floor of the tank. During this, the inlet valve to the tank is open and a return-activated sludge pump takes the mixed liquor from the outlet end of the tank to the inlet to "seed" incoming sewage with live bacteria.
The dual tank arrangement allows one tank to be in settle/decant mode whilst the other is aerating and filling, thus facilitating continuous treatment. HRT (hydraulic retention times) and aeration durations vary according to plant size and the composition of the incoming wastewater, but are typically 60 - 90 minutes. Addition of oxygen to the liquor encourages multiplication of aerobic bacteria which consume the nutrients; this encourages production of nitrogen compounds as the bacteria increase their number (a process known as nitrification).
The settling stage (usually the same length of time as aeration) allows sludge formed by the bacteria to settle to the bottom of the tank, and the aerobic bacteria continue to multiply until the dissolved oxygen is all but used up. Conditions in the tank, especially near the bottom, are now more suitable for anaerobic bacteria to flourish, many of which - and some of the bacteria that prefer an oxygen environment - now commence using nitrogen as a base element, extracting it from the compounds in the liquid and using up the nitrogen compounds created during aeration. This is known as denitrification.
As the bacteria multiply and die, the sludge within the tank increases over time; a waste-activated sludge pump removes some of the sludge during settlement to a digester for dewatering, normally by a belt press or centrifuge. The quantity or "age" of sludge within the tank is closely monitored, as this can have a marked effect on the treatment process. The sludge is allowed to settle until clear water is on the top 20%-30% of the tank contents. Generally, a submerged weir decanter is used to remove the clarified wastewater.
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