What is a Centrate?
Centrate: Liquid separated from sludge in a centrifuge
Centrate is the liquid that leaves a centrifuge when sludge is dewatered; the equivalent from a belt or plate press is called filtrate. Because the sludge has usually been through anaerobic digestion, centrate is warm and rich in ammonia and phosphorus, and although it is a small fraction of the plant’s flow it can carry 15 to 30 percent of the nitrogen load when it is returned to the head of the works. Plants with nutrient limits therefore treat this sidestream separately, with processes such as deammonification (anammox), or recover the phosphorus as struvite. Klarifi tracks 102 US plants reporting nutrient removal and 1,548 with anaerobic digesters, the plants where centrate matters most.
Why a small flow is a big load
Digestion converts organic nitrogen in the sludge into ammonia, and dewatering sends that ammonia back in the centrate. Returned in a few hours each day, it arrives as a slug that the aeration basins must nitrify, raising energy use and, at plants with tight limits, risking permit violations. Phosphorus behaves the same way and, combined with magnesium and ammonia, forms struvite scale in pipes and centrifuges.
Sidestream treatment
Options include equalising the return flow so it arrives evenly, biological sidestream processes that remove nitrogen with far less oxygen than the main plant (deammonification with anammox bacteria is the most common), and struvite recovery reactors that turn the phosphorus into a saleable fertiliser while preventing scaling. These projects are typical follow-ons at plants that have added digesters or received new nutrient limits.