ANAEROBIC DIGESTION OF DAIRY FACTORY EFFLUENTS
Report No 455/1/01
BACKGROUND AND MOTIVATION
Some years ago, it was estimated that the dairy industry consumes approximately 4.5 million m3 water per annum in over 150 dairies in South Africa (Water Research Commission, 1989). Between 75% and 95% of the water intake volume is discharged as effluent. South African dairies and dairy factories received and processed approximately 1.86xl06 kl milk during the 1989/1990 year (Dairy Board, 1990). Milk and milk products have exceptionally high chemical oxygen demand (COD) values (milk: 218 000 mg.1-1, skimmed milk: 100 000 mg.1-1, whey: 80 000 mg.1-1) and the inevitable wastage of milk and milk products contributes greatly to pollution loads discharged by dairies. The average COD of dairy effluents is approximately 3 800 mg.l-1.
While most larger dairy factories dispose of their effluent into municipal sewers, cases of effluent disposal into the sea and disposal by means of land irrigation do occur. In contrast to this, most smaller dairy factories dispose of their effluent by irrigation onto lands or pastures. Surface and ground water pollution is therefore a potential threat posed by these practices.
The aim of this study was to firstly determine the extent of effluent related problems experienced by the dairy industry in South Africa. Secondly, the feasibility of using the anaerobic hybrid digester for the treatment of dairy factory waste water had to be determined. Finally, the aim was to assess the hybrid digester as a treatment option for effluents emanating from three dairy factories producing different types of dairy products.
The objectives of this research programme were as follows:
RESULTS AND CONCLUSIONS
Effluent production and disposal in the South African dairy industry: A postal survey
In South Africa, where water has been identified as the country's most important natural resource, the dairy industry is significant, both from a water intake and discharge point of view. The requirements of the dairy industry in relation to on-site effluent treatment were thus determined by means of a postal survey. Of the 247 questionnaires sent out, 81 were returned. The data obtained indicated that the respondents from the survey receive and process 70% of the total milk production in South Africa. A diverse range of effluents and effluent problems were described by the respondents. The survey results indicated that larger factories generally dispose their effluents into municipal sewer treatment works resulting in high disposal costs. The majority of smaller factories and dairies dispose of their effluents by means of irrigation onto lands and pastures. A possible side-effect of this practice is ground-water pollution. Most of the respondents expressed a need for more information on the subject and a proposed project for the development of a biological effluent treatment procedure was supported by 49% of the respondents. These respondents represent 40% of the total milk volume processed in the country. The supportive respondents were also responsible for 84% of the reported municipal levies.
Anaerobic treatment of a synthetic dairy effluent using a hybrid digester
A mesophylic laboratory-scale hybrid anaerobic digester, combining an upflow sludge blanket and a fixed-bed design, was evaluated for the anaerobic treatment of a synthetic dairy effluent. In the first experimental study, the chemical oxygen demand of the dairy effluent was increased stepwise from 3 700 to 10 300 mg.l-1. In the second experimental study the COD of the synthetic dairy effluent was kept constant at 10 000 mg.l-1 and the hydraulic retention time was shortened stepwise from 4.1 to 1.7 d. A COD removal of between 90 and 97% was achieved at organic loading rates of between 0.82 and 6.11 kg COD m-3.d-1. At an HRT of 1.7 d, the digester achieved a methane yield of 0.354 m3 CH4 per kg C0Dremoved. The best results in terms of methane yield were achieved at an HRT of 1.9 d. The data also showed that the maximum operational potential of the digester had been reached, as indicated by the drop in methane yield observed at the end of the second experimental study. The results clearly show that this particular type of digester would be suitable for the anaerobic treatment of dairy effluents. An important consequence of the data from this study is that a two-phase set-up will be required to protect the methanogens in the digester from inhibitively low pH values and high concentrations of volatile fatty acids (VFAs) produced during the acidogenic phase. A two-phase system will allow pH control in the acidogenic phase, should it be needed at a full-scale or pilot-scale treatment plant.
Two-phase anaerobic digestion of three different dairy effluents using a hybrid digesterA mesophilic hybrid digester was used in conjunction with a pre-fermentation step to treat effluents from three different dairy factories which included a cheese, a fresh milk and a milk powder/butter factory. The effluents from these factories were analyzed and the chemical oxygen demand, pH and effluent volumes were found to be highly variable over short time intervals. The pH was found to vary between 2.2 and 11.8 units, and the COD values ranged from 800 to 15 000 mg.l-1 over a period of two hours. Significant differences were also found in the composition of the effluents from the three factories. The average COD of effluents emerging from the three factories varied between 1 908 and 5 340 mg.1-1. During the anaerobic treatment of these effluents using the hybrid digester, the COD of the effluents was reduced by between 91 and 97%. The methane yield (per kg of C0Dremoved) varied between 73 and 91% of the theoretical maximum yield. The data clearly indicated that anaerobic treatment of the different dairy effluents was successful. However, it was also clear that balancing tanks will be essential in full-scale treatment plants due to the high variations in effluent quality that were found over very short intervals. It was also found that the pre-fermentation step may be unnecessary when using highly diluted effluents as a digester substrate.
Optimization of acidogenesis of dairy effluents using a selected Peptostreptococcus productus strain
The optimization of acidogenesis of a synthetic dairy effluent was investigated in this study, using a continually operated mixed-culture acidogenic bioreactor. A population study was conducted and 47 isolates were obtained. These isolates were characterized and found to be very similar in terms of biochemical characteristics. By using both the Jaccard and the Sokal and Michener coefficient, a percentage similarity of above 80% was obtained for 93% of the isolates. The isolate with the highest volumetric lactic acid productivity (Qp(max)), was subsequently identified. Three isolates had Qp(max) values below 10.0 g.l-1.d-1, while two isolates had Qp(max) values above 30.0 g.l-1.d-1. The dominant isolate, both in terms of Qp and total counts in the mixed culture, was Peptostreptococcus productus. The strain (F06) with the highest Qp(max) was subjected to a factorial design experiment, to determine the optimal levels of the factors temperature, COD and pH. With the exception of pH level, the optimal operational parameters as found in the factorial design, closely correlated with the levels as used in the acidogenic bioreactor. This underlines the highly selective pressure exerted by a chemostat on a bacterial population. In this study, an optimization procedure was developed which can possibly be used for the microbial and initial operational optimization of acidogenesis, even in existing full-scale acidogenic bioreactors. The optimal values of the various operational parameters must be verified and possibly adapted, when the pure cultures obtained in this study, are used for pilot or full-scale acidogenesis. However, the results obtained during the process optimization in this study will facilitate start-up and initial operation of such a full-scale acidogenic bioreactor. The use of a known isolate has yielded the unexpected bonus of odour control. Similar advantages, such as the elimination of unknown and possibly pathogenic bacteria are also implied.
The South African dairy industry was shown in this study, to be in a highly favourable position to contribute to water conservation in South Africa. Specific actions that the dairy industry can take to contribute to water conservation in South Africa, involve the very obvious "prevention is better than cure" approach. An environmental audit can be used to identify problem areas and it is recommended that this be the first step, to ensuring environmental compatibility of any individual factory. The logical way to deal with dairy effluents, as with any other waste, is to include the following steps in an effluent action programme:
These measures can then be taken to the point where they complement each other in the most favourable manner, both environmentally and economically. From the data obtained in this study it is clear that, due to the differences between individual factories in terms of their products and effluents, no two detailed effluent action programmes will be similar.
The impression gained from the postal survey, was that at least the first three of the above steps are applied at various levels of sophistication by the South African dairy industry. While there is certainly scope for improvement in many areas, such as specific water consumption figures, it is also believed that the current levels of technology available in South Africa for the prevention, minimization and recycling of wastes in the dairy industry, have virtually been taken to its limits in terms of cost effectiveness and practicality.
From a microbiological viewpoint, the fourth step, namely wastewater treatment, is the one outstanding area where the current state of the biotechnology can be improved considerably, without necessarily adding to the overall complexity and cost of existing processes. Bearing in mind that previously, wastewater treatment received very little specific attention from the dairy industry in South Africa, it was believed from the outset, that this study would serve a dual purpose. In the first instance, a contribution to the existing body of scientific knowledge could be made and in the second instance, the availability and practicality of existing biotechnological options for the treatment of dairy wastewaters, could be brought under the attention of the South African dairy industry.
It may be argued that dairy factories, who pay for the privilege to dispose effluent to municipal sewage treatment works, are indeed applying the fourth step. However, this can be very expensive and thus, it is unlikely to be the most economical option. More important, however, occasional concentrated effluent discharges from a large dairy factory can cause a negative environmental impact, especially if the receiving municipal treatment plant is running at or near its full capacity. This phenomenon has been observed in South Africa and Europe, the latter being described in the literature. Finally, the technical requirements for the treatment of domestic sewage sludge differ markedly from those for the treatment of dairy factory effluents. Moreover, dairy factory effluents in itself show vast differences, in terms of composition and overall character, between the different types of factories. An on-site effluent treatment plant can be tailored to the specific demands of that particular effluent and thus offer improved cost-effectiveness to the factory as well as reliable organic overload protection to the municipal treatment plant.
Laboratory-scale anaerobic digestion
The hybrid anaerobic digester used in this study was deliberately chosen as subject for the experimental investigation of the anaerobic digestion of dairy factory effluents, as it has previously yielded excellent results during the treatment of "difficult" wastes such as bakers' yeast factory effluent and landfill leachate. In addition, it was also clear from the literature review that the hybrid digester had never before been used for the treatment of dairy factory effluents.
The very positive results from the anaerobic treatment of the three different types of effluents, underlines the suitability of the synthetic effluent to serve as a laboratory replacement substrate in effluent treatment trial runs.
Optimization of acidogenesis
Pre-acidification was found to be a pre-requisite for the successful high rate anaerobic treatment of dairy factory effluents. Prior to this study, the continuous pre-acidification of dairy factory effluents has never been studied with microbial strains, selected specifically to optimize acidogenesis during two-phase anaerobic treatment. The optimization of the pre-fermentation process in general, was considered essential in order to ensure the successful practical application of the process.
It is believed that, based on the data obtained in this study, acidogenesis of virtually any dairy effluent can be microbially optimized by studying the naturally occurring bacterial population in an operational acidogenic bioreactor. Furthermore, this process can be replicated at any scale. By using the capabilities of naturally occurring but specially selected micro-organisms, it is believed that any waste treatment biotechnology can be developed in an elegant and cost-effective manner. In this study, the most suitable bacterium was employed under the most favourable set of operational parameters, primarily for the production of organic acids. An unexpected bonus was odour control. Other benefits include the control of pathogens and possibly also bacteriophage. It is believed that the factorial design experiment, as used in this study, provides a starting point for the optimization of all types of dairy factory effluents.
RECOMMENDATIONS FOR FUTURE RESEARCH
This study is not the final and complete answer to the effluent problems of the South African dairy industry. It does, however, provide a starting point from which the dairy industry can proceed to make an informed decision, based on viable effluent treatment options. When anaerobic digestion is considered as an option for the on-site treatment of dairy effluents, the results of this study will hopefully remove many fundamental uncertainties.
However, several measures should still be taken to ensure that the application of on-site anaerobic treatment for the treatment of dairy effluents is as successful and efficient as possible.
It is likely that channeling may have occurred, as the digester was continually operated for more than two years. A tracer test and a residence time distribution (RTD) analysis will describe the mixing characteristics of the digester and thus identify so-called dead spaces. This will have important consequences and is, therefore, considered as a prerequisite for successful pilot-scale work.
Research of a more fundamental nature is also justified by the results obtained during this study. From the literature it was seen that the kinetic constants of acidogenesis in synthetic dairy effluents are well established. Furthermore, in this study, a very important contribution to the understanding of acidogenic bacterial populations, was made. Methanogenesis, the key aspect of the anaerobic digestion process, remains open for further investigation. Due to the unique configuration of the hybrid digester and the two-phase set-up which was used in this study, the biochemical kinetics and the microbial dynamics of the methanogenic bacterial population in the hybrid digester, may prove an important study field.
The current state of waste-treatment biotechnology will allow successful on-site treatment of dairy factory effluents. However, further research of both applied and fundamental nature, should not be neglected since it can only enhance the efficiency and practicality of the process, thereby rendering it more attractive to the dairy industry and thus increasing the probability that the results of this study will eventually find a practical application.
Evaluation of contract objectives
The first contractual objective was to survey the South African dairy industry and the successful results of this part of the study were published in Water SA in 1993. As a result of this survey, many members of the dairy industry in South Africa were made aware of the potential of anaerobic digestion, and many enquiries were received.
The investigation into the use of anaerobic digestion as an effluent treatment option for the dairy industry, was carried out in two phases. The first dealt with a synthetic effluent, and the basic operational parameters were established. The second part had a more practical approach in the sense that actual effluents were used as digester substrate. The results of these two studies, are considered to meet the second objective as listed above, and the results from both phases have been published in Water SA.
The ADUF part of this laboratory scale project was not successfully carried out, due to unsurmountable technical difficulties. The minimum reactor size required for successful ADUF work is around 250 litres, and the biggest digester available at the Institute had a capacity of only 97 litres. Initial trials using this digester proved unsuccessful and results are not reported.
Thus, instead of reiterating previous small-scale ADUF failures, it was decided to reschedule the ADUF work and to include it in the follow-up pilot-scale project where it could be carried out on a more practical scale. A project proposal (7he complete treatment of dairy factory effluents by means of primary anaerobic digestion and secondary algal production") was subsequently approved by the WIRC and work will start in January 1996. This constitutes the only deviation from the original aims as set out in the contract.