New Sludge Process Produces Class A Biosolids

Waukesha, WI---Class A biosolids can be produced from existing wastewater treatment operations using a fully automated, pre-conditioning sludge process applied before anaerobic digestion, according to an announcement from Envirex.

The Waukesha, WI-based company explained that Class A biosolids can be land applied without use of access restrictions, unlike traditional dewatered sludges which require regulation and monitoring. The conversation to Class A biosolids offers significant savings in sludge disposal costs to most wastewater facilities.

The new process allows existing facilities to continue to produce bio-gas (methane) that is converted into heat of electricity. Other benefits are said to include the capability to significantly increase the capacity of existing digesters and to greatly enhance the dewaterability of biosolids.

The new process provides aerobic thermophilic digestion pretreatment that disinfects and conditions the sludge. It is used ahead of new or existing anaerobic digestion which stabilizes the organic material and provides an environment for bio-gas production. The operating temperature in the pretreatment reactor is held at about 149 F. That temperature, and a minimum retention time of one hour, reduces the level of harmful pathogens to less than the levels required by the 503 Regulations.

During operation, sludge thickened to three to seven percent solids is fed into a stainless steel pre-conditioning reactor in small batches. In the reactor, sludge is heated and intensely mixed via a hyperbolic agitator. After being pre-conditioned, the sludge is pumped through a sludge -to-sludge heat exchanger, to cool before entering the anaerobic digester and to concurrently preheat incoming sludge.

The entire process is fully automated. PLC’s monitor tank levels, temperatures and cycle times. The reactor and heat exchanger for the new process require minimal space and normally can be easily sited. Operating costs are low because of the specially insulated reactors an heat exchangers; the continual small batch feeds; and the efficiency of the sludge to sludge heat exchanger. Energy costs are said to fall in the range of $1.30 to $4.50 per ton of dry solids.