On treatment systems with aerobic digestion, it is possible to continue to dose the biosolids in the digestor, further breaking down and reducing the amount of solids to be disposed of.

On several applications in the United States, this has resulted in clients not having to haul solids off site for five years consecutively.

Case Study: Reduction of Biosolids

DESCRIPTION OF PLANT:  This wastewater treatment plant (WWTP) treats about 0.6MGD.  The plant was designed with two parallel aeration basins with secondary clarifiers.  Biosolids handling originally used holding tanks for wastage with an anaerobic digester.  This was replaced by a chemical conditioning technology called BCR with the intent of producing a Class A sludge. In 2016, ABS was invited by the client to implement a Biofermentation^® Program on the aeration basin and subsequently to evaluate a process ABS developed called “Advanced Digestion” to further reduce the amount of biosolids handled.
 
OPERATIONAL CHALLENGES:  Pre-2009, the City operated an anaerobic biosolids handling process producing about 1,200,000 gallons per year of a Class “B” sludge with costs in the region of $162,000 per year ($13,500/month).  The client decided to implement the BCR Process in order to have more flexibility with biosolids disposal by producing a Class A sludge.  After investing approximately $2MM in the BCR Process the client found that the process was unable to meet Class A on a routine basis, required expensive maintenance and required a “full-time” operator resulting in excessive cost of operations in the region of $150,000-240,000 per year.  .
 
PROJECT DURATION: The Program started in April 2018 and is ongoing.
 
PROJECT OBJECTIVES: To evaluate reduction the quantity of biosolids produced.
 
FEED POINT: Directly to the digester.

RESULTS: ABS started a program of dosing the digester routinely in April 2018 along with cycling the air on/off which ultimately saved the City 70% in energy costs versus operating the digester aerobically as designed.  The resulted in no hauling of biosolids over the past 2 years as shown in Figure 1 with a reduction of 90% across the plant. 
 
As phosphate is a conservative element it must leave the wastewater treatment plant either in the biosolids or effluent.  For plants with phosphate limits, ABS estimate that around 80%+ could be achieved without the need for chemical phosphate removal of the digester decant.  However, with chemical pretreatment of decant prior to returning to the head of the aeration plant, ABS believes 90%+ could be achieved with only removal of inorganic/non-biodegradable solids.