Wastewater treatment

Sludge screening saves maintenance

Anaerobic digestion plants and sewage treatment plants can lower maintenance costs by screening sludge at the primary or secondary stage of treatment.

Sludge screening results in a more efficient digester operation, with up to 20% more gas production from anaerobic digesters. As well, biosolid quality is improved as inorganic impurities are removed, which leads to increased marketability of the biosolids for reuse.

“The practice of sludge screening is a new concept for the Australian market, but numerous advantages have been demonstrated by the more than 700 installations of the Huber Strainpress worldwide,” said John Koumoukelis, National Sales Manager - Industrial/Municipal for Eimco Water Technologies-AJM Environmental Services. EWT-AJM supplies the Strainpress sludge cleaner from Huber Technology of Germany.

“Many of Australia’s major water authorities, including the Western Australian Water Corporation and SA Water, have recently included the Strainpress in their plant upgrades,” he says.

The Strainpress sludge cleaner is an inline screen that removes coarse and fine material from sludge streams. The removal of screenings and scum helps protect digesters as well as plants for dewatering, stabilisation and drying.

Applications include primary sludge, mixed primary and secondary sludge, secondary sludge, and scum from primary clarifiers. The system is designed to treat flows from 10 – 60 m³ per hour, with feed solids from 0.5 – 6.0% w/w solids.

Typical removal rates are between 100 – 200 L of screenings per 1000 L of sludge, with the removal efficiency dependent upon the type of screens installed. Screenings are dewatered up to 45% solids.

Installation of the cleaner into an existing closed pipe system is easy, as Strainpress is an inline pressurised screen that operates automatically.

Installations have shown much more efficient digester operation, with up to 20% more gas production in anaerobic processes. There are also significant reductions in maintenance costs of digesters, particularly in the cleaning of recirculation pumps and heat exchangers.

“Even with the use of pre-treatment screens with an aperture of 6 mm or less in conjunction with grinders, fine material still passes through the inlet works to the primary sedimentation stage,” Koumoukelis said.

“Over a period of time, this material grows significantly in size to the point where it forms a long strand of hair, tightly bound. This material causes blockages to digester equipment, resulting in labour-intensive periodic maintenance and a loss in digester efficiency.

“With the use of Strainpress, sludge grinders can be done away with altogether. Improved bio solid quality, due to the removal of inorganic impurities, means bio solids are more marketable for reuse,” he said.

The digesters experience a reduction in maintenance costs and an increase in efficiency. Sludge-handling plants using decanter centrifuges and belt filter presses also benefit.

Homogenisation of the sludge assists in polymerisation, helping the subsequent dewatering process. Accumulation of screenings in belt-press doctor blades is eradicated, allowing for better cake release and less maintenance.

Typically, the Strainpress is installed upstream of digesters. The unit can be installed in the primary sludge line from the primary sedimentation tank to the digester.

Linking the machine to the primary sedimentation raw sludge pump means it operates simultaneously, eliminating the need for secondary pumping. For example, if primary sludge is drawn for six minutes every hour, then the Strainpress operates for the same period.

The headloss through the unit is 0.4 – 0.6 bar, which is small enough to enable direct inline installation without any impediment to raw sludge pumps.

The Strainpress sludge cleaner consists of an inlet screening section, a pressing section and a screenings discharge cone.