Garbage in, garbage out?
There has been a lot of publicity lately regarding sludge. Whether it is the energy we produce from it, how we dispose of it, recycle it or just handle it, it is - rightly - a subject of interest.
Recent estimates suggest that it could be possible to generate enough energy to power over half a million homes if every bit of sludge was utilised. However, the methods we use to separate it, collect it, the efficiency of the systems and how we keep it running has come under great scrutiny. You may be aware of the statistics that show that between 50% and 65% of all energy consumption on a typical activated sludge wastewater treatment plant is devoured by the aeration system.1,2,3 Figures from recent reports suggest the traditional methods of secondary treatment consumes 68 GWh annually in the U.S. alone. This is enough to power another twenty thousand homes.
It seems to me as if we are jumping a step here: surely, to get the most benefit out of the sludge, the aeration system, and indeed the entire treatment chain, you need to provide these processes with the best quality of sludge you can.
The presence of non-organic material (or grit and rag, to be specific) in activated sludge not only takes up potential energy production volume, but reduces the capacity and treatment volume of the whole system.
If we ignore the other elephants in the room such as the abrasive and clogging effects of grit and rags on mechanical equipment such as pumps, bearings and conveyance systems, downtime and loss of capacity for clean-out and maintenance, consider the insidious impairment of grit on aeration and Return Activated Sludge systems. We have calculated that for every one percent of grit that accumulates in an aeration tank, an extra one percent of energy is required to push the air through the liquid (see A giant cost lurking in aeration basins by Paul Barter).
We must also consider that a significant amount of imported sludge is injected into the treatment chain downstream of the headworks. This leaves the door wide open for deposition in the digesters. A paper published in 2010 reported that grit and rag could account for 40% of the effective treatment capacity in a digester. This is generally from either inefficient headworks or untreated sludge from small works without this facility.
As with most things in life, prevention is better than cure and we must become more energy conscious. This starts at the very beginning of the treatment chain.
After all, you wouldn't knowingly fill your car with dirty fuel, would you?
Keith Hutchings, Hydro International
1 US EPA (1999) Wastewater Technology Fact Sheet – Fine Bubble Aeration
2 Metcalf and Eddy Inc. (1991) Wastewater Engineering, Treatment, Disposal, Reuse 3rd ed., McGraw-Hill, New York. p1253
3 Metcalf and Eddy Inc. (1991) Wastewater Engineering, Treatment, Disposal, Reuse 3rd ed., McGraw-Hill, New York. p457