Degasification of mixed liquor by subjecting it to vacuum is a physical process used in biological nutrient removal (BNR) to improve settleability and allow for achieving higher mixed liquor suspended solids (MLSS). Vacuum degassing installation is located between the last cell of the bioreactor and secondary clarifiers. In this process two operations are performed: gas bubbles contained in mixed liquor leaving the bioreactor are removed and concentration of gasses (mainly nitrogen gas) dissolved in the liquid is reduced. Lack of gas bubbles and concentration of dissolved nitrogen gas below saturation in mixed liquor significantly improved sludge settling in secondary clarifiers and eliminated floating scum formation. Presented settleability tests of degasified MLSS and return activated sludge (RAS) from various BNR facilities showed continued settling and/or thickening for over 3 h at room temperature, without exhibiting any solids separation. Settleability tests of biomass that was not degasified typically led to flotation of portion of the sludge after about 1.5 h. Plants equipped with vacuum degasification consistently operate at larger than typically recommended final clarifier sludge surface loading rates. Rates as high as 180-220 kg TSS/m2d and deep sludge blankets have been employed. Such plants were shown to maintain operational levels of MLSS at 4500 to 6000 mg/L and higher.

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