Anaerobic treatment of slaughterhouse wastewater using the UASB process

Effluents from the slaughterhouses, meat and poultry industries are heavily polluted and contain a high concentration of biodegradable organic materials. Therefore, the pollution capacity of these industries is high. Most of these industries discharge their effluents to a sewer or a watercourse.
In order to comply with water pollution control standards and to reduce costs on sewer surcharges, these industries have to apply an adequate treatment of their effluents.
Physical and chemical treatment methods as well as the conventional biological treatment processes are frequently applied in the treatment of these effluents. A combination of the methods are required where the effluent is to be discharged to surface waters, since no single treatment method will provide sufficient effluent.
In the last decade, the high rate anaerobic wastewater treatment systems have become a good alternative for conventional aerobic as well as anaerobic biological treatment methods. The high rate anaerobic treatment systems were initially developed for the treatment of highly soluble low and medium strength wastewaters. These systems provided only a partial treatment of complex wastewaters containing a high fraction of suspended solids such as slaughterhouse wastewater.
Investigations have shifted towards the application of high rate systems like the upflow anaerobic sludge blanket (UASB) for the complete treatment of agro-industrial wastewaters which are more difficult to handle, because they contain relatively high concentrations of suspended solids, i.e. complex wastewaters.
Presently, the UASB system is the most widely applied high rate anaerobic system for complete treatment of such complex wastes.
This thesis focuses on the question whether, and under which operationa conditions and environmental circumstances a one stage UASB mesophilic anaerobic treatment system is suitable for a complete treatment of slaughterhouse wastewater in practice.

The feasibility of using the upflow flocculent anaerobic sludge blanket (UASB) process for a one stage anaerobic treatment of unsettled complex slaughterhouse wastewater, which contains approximately 50% of coarse insoluble COD, was investigated (Chapter 2). The continuous experiments were performed in a 25.3 m ³UASB pilot-plant which was operated under semi-continuous conditions, viz. with a varying organic load over day and nighttime (i.e. high organic load during the daytime and with low organic load at night) and with weekend feed interruptions. The UASB pilot-plant was operated at a temperature of 30° C. In order to assess the feasibility of the process under conditions of lower temperatures the temperature was reduced to 20°C, 20 weeks after the start-up of the reactor.
The data indicated that the system can satisfactorily handle organic loads up to 3.5 kg COD m ^-3day ^-1at a liquid retention time of 8 h at temperatures as low as 20°C. Temporary shock loads up to 7.5 kg COD m ^-3day ^-1during the day time at a liquid retention time of 5 h were accommodated satisfactorily provided such a shock load was followed by a period of low loading, e.g. at night.
A significant discrepancy was found between the treatment efficiency in terms of COD reduction and to the lower calculated percentage of supplied COD _total converted into methane-COD. This difference indicated that a significant portion of the achieved COD reduction was due to the accumulation of non- or slowly biodegradable substrate ingredients in the reactor. No differentiations could be made between the different types of substrate ingredients that accummulated in the reactor because the accumulated sludge was not characterized. However, a part of the accumulated substrate was converted to CH ₄ in periods of feed interruptions.

In Chapter 3 the feasibility of the upflow granular anaerobic sludge blanket process for a one-stage anaerobic treatment of slaughterhouse wastewater was investigated. The experiments were performed under semi-continuous operational conditions viz. continuous feeding at a constant organic load (24 h day) during the working days but with weekend feed interruptions, and process temperatures of 30°C and 20°C. Under a stable operation of the system, i.e. at a maximum COD reduction and a high conversion of COD into methane, the optimal loading rates that could be applied were 11 kg COD m ^-3day ^-1and 7 kg COD m ^-3day ^-1at 30°C and at 20°C respectively.
The system was less effective in the removal of coarse suspended solids, compared to the removal of the colloidal and soluble fractions from the slaughterhouse wastewater.
The data obtained in these investigations indicate that imposed prolonged loadings exceed the optimal loading rates, lead to deterioration of the specific methanogenic activity of the sludge, due to the accumulation of colloidal and soluble fractions of the wastewater in the sludge bed. Therefore, it was concluded that the system stability strongly depends on the processes involved in the removal of the colloidal and soluble compounds from the wastewater and their conversion into methane. As the predominant - non-biological-mechanisms underlying the elimination of these wastewaters pollutants were considered the entrapment and the adsorption mechanisms. The effect of these mechanisms on the rate of the liquefaction of the accumulated substrate - which is the required first step in their conversion into methane - were discussed.

The different pollutant fractions of the wastewater, viz. the coarse suspended solids, the colloidal and the soluble compounds affect the performance of the UASB reactors because of the different mechanisms involved in the removal of these substrate ingredients and their subsequent conversion into methane. Therefore, these mechanisms were investigated in more detail. The results of these investigations are presented in Chapter 4. The experiments were performed in a one-stage flocculent sludge UASB-reactor under continuous operational conditions viz. continuous feeding at a constant organic load during 24 h a day and 7 days a week.
The COD removal efficiency of the UASB reactor exceeded the COD removal efficiency as expected from the observed CH ₄ production, indicating once again that non-biological mechanisms are involved. Two different non-biological mechanisms were distinguished in the removal of substrate ingredients from the wastewater. The entrapment mechanism prevailed in the elimination of coarse suspended solids, while mainly adsorption mechanisms are involved in the removal of colloidal and the soluble fractions of the wastewater.
A continued accumulation of substrate ingredients in the reactor ultimately will become detrimental for the stability of the anaerobic treatment process, as it leads to sludge flotation and consequently could result in a complete loss of the active biomass from the reactor.

After having demonstrated the principle feasibility of the upflow anaerobic sludge blanket (UASB) process for a one-stage anaerobic treatment of the slaughterhouse wastewater, we decided to assess the maximum possible extent of anaerobic degradation of the soluble, colloidal and coarse suspended solids fractions of the slaughterhouse wastewater (Chapter 5). In this way we intended to get a better insight in the real limitations of the system. All the experiments were performed at process temperatures of 30°C and 20°C, using membrane filtered wastewater (wastewater _mf ), paper filtered wastewater (wastewater _pf ) and total wastewater. The experiments were performed in a recirculated batch digester system with granular sludge. The experiments with the coarse suspended solids separated from the wastewater were performed with granular sludge as well as with flocculent sludge using conventional batch-fed stirred digesters. The maximum biodegrability percentages (i.e. conversion into methane) found at 30°C were 75% for wastewater _mf , 61% for wastewater _pf and 67% for wastewater _total while at 20°C these values were 72%, 49% and 51% respectively. The maximum biodegrability of the coarse suspended solids fraction of the waste amounts to 50% at 30°C and 45% at 20°C.
The mechanisms involved in the removal of the soluble and colloidal fractions of the slaughterhouse wastewater were thoroughly studied and elucidated. The data obtained in these experiments indicate that the prevailing mechanism in the removal of the soluble but especially also the colloidal fraction of the wastewater is an adsorption mechanism. The relatively high degree of adsorption of the colloidal fraction of the wastewater to the surface of the sludge, in combination with its high fat content, will deteriorate the specific methanogenic activity of the sludge. The adsorption of the colloidal materials will ultimately result in an enclosure of the granular sludge bacterial matter with a film of increasing thickness, and perhaps also density, which increasingly will hamper the supply of substrate to the bacteria present in the grains. The deterioration effect of fats towards the methanogenic activity of the sludge was explained on the basis of the inhibitory effect of the long-chain fatty acids of the neutral fats.
As the extent of adsorption is very similar at lower and higher temperatures, but the rate of liquefaction of adsorbed compounds drops significantly at decreasing temperatures, it will be evident that the process can withstand considerably lower loading rates at 20°C as compared to 30°C.
Therefore, it is concluded that the rate of liquefaction of the adsorbed insoluble colloidal fraction of the wastewater is the controlling factor with respect to loading potentials of the process and consequently that the temperature is the factor of predominant importance.