pH adjustment in anaerobic digestion |
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Authors: | M.G. Capri G.v.R. Marais |
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Affiliation: | Department of Civil Engineering, University of Cape Town, Private Bag, Rondebosch, Cape Province, South Africa |
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Abstract: | In the pH range 6·0–7·5, the pH in anaerobic processes is controlled by the interaction of the carbonic system and a net strong base. The acid-base state of a digestor can be monitored by only measuring pH and CO2 partial pressure. Shock doses of strong bases and carbonates causes temporary undersaturated CO2 conditions and excessively high pH. Bicarbonate dosing leaves the CO2 solubility equilibrium unchanged. In the absence of a CaCO3 precipitation inhibiting agent. CaCO3 solubility limits the pH, and Ca(OH)2 dosing is unable to raise the pH significantly. Orthophosphates inhibit CaCO3 precipitation. With [PO4] > 1·0 × 10−3mole·1−1. CaCO3. precipitation is partially inhibited. Ca(OH)2 dosing being approximately 45 per cent effective for doses up to 15000 mg 1−1 as CaCO3. At [PO4] < 1·0 × 10−3moles·1−1 orthophosphates eventually precipitate out during Ca(OH)2 dosing, thus removing the inhibition mechanism: pH is then limited by the CaCO3 solubility. Most wastes contain [PO4] > 2·0 × 10−3moles·1−1 making pH adjustment with Ca(OH)2 possible to a pH of about 7·2 although the dosages will be very high. The pH changes in a process following dosing can be predicted by the graphical representation of the carbonic and net strong base systems. |
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