Sludge line

Re-circulation sludge pumping station

The sludge re – circulation is a very important plant stage.

The sludge is pumped from a collecting sump to the biological oxidation tank (or to the nitrification tank) continuously. This sludge provides to keep the bacteria mass well balanced  and it makes their regular and intense  growth possible, independently of the organic matter content in the  water which enters the oxidation tank.

During the start-up of the plant, when a quick bacteria growth is required, the sludge is totally pumped back.

When the plant achieves a good steady state,  most of the sludge is pumped back while the rest, the excess sludge, should be treated and thickened (various methods are available )  before being discharged in a controlled dump.

The re-circulation pumps can also be used for periodical excess sludge pumping off.

Storage, Polyelectrolyte Feeding and Thickening of Excess Sludge

Before a further treatment of dewatering and compacting (i.e. filter press, belt type or frame type), the excess sludge should be thickened in a storage tank (so called thickener) where a reagent, cationic polyelectrolyte, is dosed to improve the thickening process.

After mechanical filtration sludge shows low water content and may be transported and stored in sanitary landfill.

Figure 1: Sludge after dewatering with filter press