A Water Treatment Plant By-product

March 6th, 2002

W.r. Walker P. Eng. President Walker Engineering inc.

What are Bio-solids?
The solid component remaining after the waste stream has been separated into liquid and solid streams. It usually take the form of a sludge, which can vary in concentration from 1% to 3% solids.

Source of Bio-solids

• Sedimentation Chambers in the traditional flocculation / sedimentation / filtration / disinfection process
• From the water used to backwash filters
• In the case of slow sand filters, from the periodic cleaning of the surface layer of the sand.

Traditional Water Treatment
Four Stages
 

• Flocculation
• Sedimentation
• Filtration
• Disinfection
   

Sedimentation
Settles out of the particles which have formed from the application of the coagulant eg, aluminum sulphate (alum) and / or poly-electrolyte. The settled solids and an amount of water are withdrawn on a continuous or intermittent basis.
 

Filtration
The particles which have not been removed by sedimentation are largely captured in the filters, which allow the water to flow thru layers of specially engineered sands. Periodically, the filters are isolated and cleaned by directing water through them in the opposite direction. Backwashing is tricky the rate at which the cleaning water flows must be sufficient to clean the filters but not so great as to disturb the layering of the filter media. Generally the backwash rate is considerably higher than the normal filtering rate. Filter runtimes between backwashing may be from one day to several days. The amount of water used in the backwashing operation can be in the range of 2% to 5% of the filter thruí-put. Backwashing time can be typically 30 to 45 minutes.
 

The Combined Waste Stream

The Waste Stream from a typical Water Treatment Plant has, therefore, two components;
 

• The waste drawn from the sedimentation tanks, and
• The backwash water
   

Treatment for the Combined Waste Stream
The two waste streams are usually directed to an equalization tank - little more than a container which accommodates the flows as they occur and allows for treatment at a slower rate on a 24 hour basis.

The Liquid Stream
The supernatant - or liquid - at the top of the equalization tanks is usually drawn off and treated to achieve Ontario Drinking Water Standards before being released to the environment ñ a steam, a roadside ditch, for example.
 

This leaves the tricky stuff - the sludge- or use the fancy name "Biosolids".

Disposing of the Biosolids - Options

• Concentrate using a thickener or centrifuge, truck thickened sludge to landfill
• Direct the sludge to a municipal sewer system
• Drying Beds
• Freeze-Thaw lagoons - up to 30% solids
• Most options need a landfill for final disposal