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Analysis of Centrate Composition and Evaluation of its Applicability as a Nutrient Supplement to Irrigation Water
AdvisorMarchand, Eric A.
Civil and Environmental Engineering
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Experimental analyses were carried out to evaluate the chemical composition of centrate originating from Truckee Meadows Water Reclamation Facility (TMWRF, Reno, NV) and its applicability as a nutrient supplement to reuse water used for irrigation. Centrate, the liquid fraction produced from dewatering of anaerobic digester sludge, is generally returned to the head of the treatment facility and contains up to 30% of the plant nitrogen loading. Regular chemical analyses were performed on both centrate and reuse water on a monthly basis for 16 months (August 2007 - November 2008). Tests revealed that centrate contains large amounts of the three most important macronutrients required for plant growth. Average nitrogen, phosphorus, and potassium concentrations in centrate were 1,000 mg/L, 225 mg/L, and 200 mg/L, respectively. Elemental analyses of select cations (e.g., sodium, iron, magnesium, and calcium) and potential metals of concern (e.g., aluminum, cadmium, cobalt, and copper) revealed that their concentrations in centrate would not cause reuse water to exceed published recommended maximum irrigation levels if supplemented with centrate. In addition to conducting chemical analyses, centrate was subjected to bench-scale disinfection with chlorine, ozone, and UV radiation to identify the disinfectant doses required to achieve complete inactivation of coliform bacteria. Total and fecal coliform were used as indicator organisms and were enumerated following disinfection tests. Chlorination tests showed that inactivation of coliform bacteria in undiluted centrate was achieved with a dose of 75 mg/L of chlorine (as Cl2) after a 30 minute contact time. Changes in centrate TDS were monitored following chlorination and TDS values were found to increase by up to 40%. Inactivation of coliform bacteria in centrate by ozonation required a dose of 46.8 g/m3; a fairly high value considering that typical doses for wastewater are in the 16-40 g/m3 range. An effective UV dose of 324 J/cm2 was required for coliform bacteria inactivation by UV radiation. This is an extremely high dose considering that reasonable doses are those less than 200 mJ/cm2. Formation of disinfection byproducts in disinfected samples was also monitored. Chlorinated and ozonated samples were found to contain very low concentrations (less than 15 μg/L) of select trihalomethanes and haloacetic acids.