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Development of the Neutralizer® Process for Disinfection and Stabilization of Municipal Wastewater Residuals

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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 under an acid environment pH less than 3.3. 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 Process to Further Reduce Pathogens (PFRP) by EPA's PEC in the early 1990's, however, the utilization of ozone to hold the ORP at > 100 mV was very costly and made the process not viable economically. The Neutralizer® Process is able to control the ORP with chlorine dioxide, which is much less expensive and more reliable than ozone in a municipal residuals environment. Utilizing chlorine dioxide and nitrous acid under acidic conditions to meet Class A requirements for disinfection and is also able to stabilize raw sewage residuals.

The Neutralizer® Process has demonstrated disinfection of biosolids by reducing the fecal coliform and viral densities to less than detectable limits, and helminth eggs to 0% viability. 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. This process also is able to assist in the stabilization of both digested and raw wastewater residuals, as demonstrated by volatile solids reduction. With an end-product pH in the neutral range, and demonstrated residual chemical disinfection, there is potential for long-term chemical stabilization which to this point has not been achievable except with the reduction of moisture content and pH adjustment (alkaline agent addition).

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 biosolids is only 4 hours with the Neutralizer® Process, where many of the other PFRP approved processes require days to disinfect (U.S. EPA 2003). The Synox Process required at least 12 hours (Reimers et. al, 1991). Also, the resulting biosolids may have beneficial uses, such as a fertilizer or soil amendment, without the liability and human health concerns associated with Class B biosolids. The process, 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.
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Document Type: Research Article

Publication date: 01 January 2006

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