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In the late 1980's, Tulane researchers developed the Synox process, which utilized ozone, sulfuric acid and nitrous acid to disinfect biosolids (Reimers 1991). From this work, it was noted that nitrous acid was the primary disinfecting agent to inactivate helminth eggs. The process was observed to be effective and would produce a Class A biosolids within 4 to 24 h depending upon the dosage of nitrous acid in the range of 1500 to 400 mg/L, respectively. The Synox process was approved as a PFRP process by EPA's PEC in the early 1990's. The problem with this process was related to the utilization of ozone to hold the ORP at > 100 mV. The ozone process was very costly and made the process not viable economically. The Modified Neutralizer Process is able to control the ORP with chlorine dioxide, which is much less expensive and more reliable in a municipal sludge environment. This process utilizes chlorine dioxide and nitrous acid under acidic conditions to meet Class A biosolids requirements for disinfection and is also able to stabilize raw sludge.

The process has disinfected the biosolids by reducing the fecal coliform to less than detectable limits, helminth eggs to 0% viability and viral densities to detectable limits. It is important to note that the nitrous acid is excellent for the helminth eggs but the virus and bacteria need the assistance of chlorine dioxide to be disinfected to Class A criteria. Another interesting observation is the ability of the chlorine dioxide to assist in the stabilization of both digested and raw biosolids at the range of pH 7. This point enables long-term chemical stabilization, which to this point, has not been achievable with moisture content and pH (alkaline agent addition).

The Modified Neutralizer™ Process is able to generate Class A biosolids with regard to both pathogen reduction and vector attraction reduction standards. There are two main benefits to this process, as compared to other PFRP approved processes. First, by replacing the ozone used in the Synox Process with chlorine dioxide the cost of this process has significant cost saving advantages. Secondly, the time needed to disinfect the sludge is only 4 hours with the Modified Neutralizer™ Process, where as many of the other PFRP approved processes require days to disinfect (U.S. EPA 2003). The Synox Process requires at least 12 hours (Reimers 1991). Also, the resulting biosolid may have beneficial uses, such as a fertilizer or soil amendment, without the liability and human health concerns associated with Class B biosolids. The system, when used without the nitrous acid, has the potential for generating stability when used in conjunction with other methods that may cause cell lysis, such as high-speed centrifugation or vacuum drying methods.

Document Type: Research Article


Publication date: 2005-01-01

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