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    Factors influencing the rate and stability of the anaerobic digestive process : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Biotechnology at Massey University
    (Massey University, 1986) Mawson, Andrew John
    Three factors affecting the rate and stability of the methane fermentation of a readily-hydrolysable feedstock were investigated. The aim of this work was to develop improved processes and control strategies to facilitate economic treatment of industrial wastes by anaerobic digestion. A comparison was made between the performance of a continuously-fed digester and semi-continuous digesters slug fed every second day. A semi-synthetic medium with glucose as the major carbon and energy source was used and seed material was transferred between the digesters, which were operated under similar loading conditions. The continuous digester repeatedly failed even when operated at dilution and loading rates much lower than the maximum values commonly reported. In contrast, the semi-continuous units provided satisfactory performance and could be easily and rapidly recovered from retarded operation. Failure of the continuous digesters was characterised by a steady fall in volatile suspended solids concentration followed by a rapid accumulation of acetate, and was attributed to a deficiency in the medium of one or more essential nutrients. These were thought to be provided in the semi-continuous digester by lysis of acidogenic bacteria or luxury uptake from the medium. Degradation of acetic and propionic acids was investigated in batch culture. Increasing the concentration of either acid from low levels decreased the rate of utilisation of the acid, but the proposed inhibitory role of un-ionised acids was not conclusively supported. Increasing the initial acetate concentration above 1000 to 1500 mg.l-1 significantly reduced the rate of degradation of propionate added at 500 mg.l-1. When acetate was added at 2000 mg.l-1 the rate of propionate utilisation was approximately half of that when acetate was present at 500 mg.l-1 or lower. In batch culture experiments, addition of up to 3.2 mM cysteine-hydrochloride or sodium sulphide, or 4.4. mM sodium thioglycollate did not inhibit total gas production from samples drawn from the continuous digester. However thy rate of methane production in effluent samples from a semi-continuous digester was inhibited by 25 % to 30 % by addition of 3.2 mM cysteine or sulphide. Inhibition was attributed to the sulphide ion. Sodium thioglycollate did not inhibit methane production from acetate but propionate degradation was markedly reduced, with increasing inhibition noted with increasing incubation time. The work adds to a considerable body of investigation into the factors influencing anaerobic digestion and the unresolved problem of process stability in long-term operation of conventional stirred tank digesters has again been highlighted. Indicators and possible causes of process failure have been suggested and further development of these should assist in the continuing increase in the rate of treatment while ensuring acceptable working margins of safety for the process.
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    Treatment of meat processing wastewater for carbon, nitrogen and phosphorus removal in a sequencing batch reactor : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Process & Environmental Technology at Massey University
    (Massey University, 2002) Thayalakumaran, Nagalingam
    The typical New Zealand meat processing industry wastewater was treated by a laboratory scale Sequencing Batch Reactor (SBR) to determine an effective operating cycle for biological carbon, nitrogen and phosphorus removal. The Activated Sludge Model No. 1 and Model No. 2 with modifications were used to simulate the treatment of meat processing wastewater using the SBR. The average values of main pollution parameters of the wastewater were characterised as 1390 mg total COD L-1, 755 mg soluble COD L-1, 75 mg L-1 NH3 - N, 145 mg L-1 TKN and 34 mg L-1 TP. The readily biodegradable COD (RBCOD) accounts for 15 - 18% of the total COD, while the inert soluble and particulate portion were 4% each. In order to establish an effective operating cycle for the simultaneous removal of nutrients and organic carbon, different dissolved oxygen (DO) concentrations in the mixed liquor, duration of operating phases and hydraulic retention time (HRT) of a 6 h cycle were tested. The most effective cycle consisted of seven phases. The first two hours of the anaerobic period was followed by the aerobic and anoxic periods. The first aerobic period was maintained at a DO concentration of 0.5 ±0.25 mg L-1 for 1 h, the second aerobic period for 1 h at a DO concentration of 3.75 ±0.25 mg L-1 and the third aerobic period for half an hour at 0.5 ±0.25 mg L-1 DO concentration. A half an hour anoxic period followed the first aerobic period. A settling period of 0.75 h followed the third aerobic period. The last quarter of an hour was for decanting and idling. The solids retention time (SRT) was 15 d, while the HRT was 2.5 d. Greater than 99% removal of biodegradable soluble COD, NH3 - N and PO4 - P was achieved in the effective operating cycle where the TN and TP in the wastewater were reduced to 10 mg L-1 and 1.0 mg L-1, respectively. In addition the soluble COD was reduced to 98 mg L-1. The key kinetic and stoichiometric parameters for ASM 1 and ASM 2 models were determined using batch tests. The heterotrophic maximum specific growth rate, yield coefficient and the half saturation constant were 2.0 d-1, 0.63 mg cell COD (mg COD)-1 and 8 mg L-1 respectively. The maximum specific growth rate of autotrophs was 0.65 - 0.80 d-1. The anaerobic phosphorus removal stoichiometric coefficients were also determined in batch tests. During the anaerobic period, when 1 g of acetate COD was initially present, 1.48 g of PHA COD was stored while 0.48 g of P was released. The batch trials conducted using acetate to assess the influence of Mg2+ in P uptake showed that the Mg2+ could limit the P uptake and the uptake rate could be represented by Monod type kinetics. In the Monod kinetic expression the Mg2+ half saturation constant was found to be 4.7 mg L-1 The molar ratio of Mg2+ with P was 0.21 during the anaerobic period, and 0.33 during the aerobic period. The SBR performance was modelled using ASM 1 and ASM 2 models after the addition of more processes in these models. Ammonification of the soluble organic N process rate was modified in the ASM 1 model. Similarly it was necessary to add anoxic P uptake and anoxic growth processes involving PHA of Bio-P bacteria in the ASM 2 model. Glycogen storage and glycogen lysis processes of Bio-P bacteria were added in the ASM 2 model to understand the involvement of glycogen in P removal. Also a modification was performed to the storage process of poly-P in the ASM 2 model to account for potential Mg2+ limitation in meat processing wastewater treatment for P removal. During the settling period anoxic hydrolysis was assumed to be negligible. The calibrated ASM 1 and ASM 2 models in general well simulated the effluent NH3 - N, NO3 - N and PO4- P of SBR cycles carried out in distinctly different periods of time and in different batch tests. As the calibrated modified ASM 2 model was able to predict the performance of an SBR cycle conducted over a time period of three months, it was used to identify the most promising treatment strategies of the SBR performance. Variation in duration of feed cycle during the first non-aerated mixed period did not affect the effluent NO3 - N, NH3 - N and PO4 - P concentrations significantly. DO concentration of 3.75 mg L-1 during the third aerobic period instead of 0.5 mg L-1 increased the effluent NO3 - N and PO4 - P concentrations. The simulations confirmed that the operating conditions identified in a 6-h cycle period for the simultaneous organic carbon and nutrient removal are effective.