Dealing with sludge...

The main product produced at the CCMUA's two treatment plants is clean water. But like many manufacturing processes, wastewater treatment creates by-products which must be dealt with. The main byproduct of wastewater treatment is sludge.

WHAT IS SLUDGE?

Sludge is a mixture of solid wastes and bacteria removed from the wastewater at various stages of the treatment process. It can be categorized as "primary sludge" and "secondary sludge".

Primary sludge is about 4% solids and 96% water. It consists of the material which settles out of wastewater in the primary sedimentation tanks, before bacterial digestion takes place.

Secondary or activated sludge is much more liquid - about 1% solids and 99% water. Secondary sludge consists of bacteria and organic materials on which the bacteria feed. About 30% of the secondary sludge produced is returned to the aeration tanks to assist with the biological process of sewage treatment. The remaining 70% must be disposed of.

REMOVING WATER

There are various ways to dispose of sludge, but in all cases the first goal is to reduce the water content. Removing water decreases the volume of sludge. This makes storage and transportation much cheaper. If the sludge will be incinerated, removing water first also cuts costs because there is less water to boil off before the solids can burn.

The CCMUA has four sludge storage tanks at the Delaware No. 1 Water Pollution Control Facility. These cylindrical tanks are about 69 feet (21 meters) in diameter at the base, and about 32 feet (9.75 meters) high from the base to the peak of the dome on top. At the Winslow Wastewater Treatment Plant, sludge goes first to aerobic digesters.

At Delaware No. 1 WPCF, sludge is next processed in the Thickening and Dewatering Building. Secondary or waste activated sludge is first thickened in one of two gravity belt thickeners. This thickened waste activated sludge is then mixed with sludge that has been removed from the primary tanks. This mixture of primary and thickened waste activated sludge is then dewatered on seven belt filter presses.

Both treatment plants have belt presses which remove water by essentially squeezing it out of the sludge. Sludge entering the belt presses is 96 - 99 % liquid and has the consistency of dirty water. By the time it leaves the belt presses, the sludge is only 80% water and has the consistency of mud. This is "dewatered sludge cake".

Water removed from the sludge by centrifuges or belt presses is returned to the beginning of the treatment process.

DISPOSAL

The dewatered sludge is removed off site for final disposal. The Authority uses a variety of off- site disposal options including beneficial reuse such as land application and composting. Other sludge disposal options include incineration and land filling.