A study of pre-acidification reactor design for anaerobic treatment of high strength industrial wastewaters
Acidogenic activities, as part of anaerobic digestion, have been discovered since the beginning of the century. Still it was mid '60's when it was initially stated in the literature that engineered phase separation would increase stability in anaerobic reactors and possibly increase substrate digestion rates. Pioneering research in the early '70's, with the first report on two-phase digestion of sludges, came as practical proof of those past assumptions. Today phase separation is a proposed option to single-stage digestion, due to the many advantages over conventional operation. Such an application utilises the different steady-state kinetic rates in the two main bacterial groups in anaerobic digestion. Furthermore, the process benefits through differences of these two groups, in relation to changing conditions. The overall result of two-phase applications is lower operational costs, with higher treatment efficiency and energy recovery. In recent decades much research work has created a positive image for two-phase applications, compared to single-stage digestion. Still until today, many consultants in the field of anaerobic processes, are not provided with sufficient knowledge to utilise fully the potential of the twophase process. It seems often the case that leading companies in the design and construction of anaerobic plants, will design pre-acidification tanks without understanding the uncontrolled acidogenic activities taking place in them. Therefore, design is based on an empirical approach or lack of knowledge of the effects of reactor design parameters on acidogenesis. Although data on acidification of industrial wastewaters is in high demand, few studies have been carried out previously to assess the effects of the whole range of engineered reactor design parameters on acidification of industrial wastewater. Out of these few studies none has examined the whole range of design parameters on freshly collected agro-industrial wastewater. Apparently, most studies have been made on synthetic versions of wastewaters or simple compounds. Additionally since the '80's anaerobic processes have been extensively applied for the treatment of agro-industrial wastewaters. Obviously the extent of information provided from this study, was particularly required to clarify many issues related to the role of acidification in the pre-treatment of agro-industrial wastewaters. The research project presented in this thesis is based on a 3-year laboratory study. Some early conclusions of this study have been presented previously in a number of papers on preacidification discussing design guidelines, advantages of two-phase applications and methods to assess acidogenesis. This thesis is focused on the complete range of findings related to the effects of various reactor design parameters, namely: temperature (from ambient to thermophilic); pH (from 4.5 to 7.0); HRT (from 6 to 12 hrs, with and without variations in the organic loading rate); addition of commercial micro-nutrients; and mixing the reactor contents. The two wastewaters studied are slaughterhouse, collected fresh each week; and synthetic instant coffee production. They are both considered as high strength wastewaters. Slaughterhouse wastewaters are found everywhere, as they are connected with daily human activities, while they are easily biodegradable wastewaters for high-rate digestion. On the other hand instant coffee production wastewaters, although not a common global industrial activity, involves more complexity for high-rate digestion, due to various recalcitrant and inhibitory compounds present in the composition of coffee. Results are based on analyses for: VFA concentration and composition (Acetic to Caproic acid). Tot. and Filt.COD, Tot.BOD, TS, VS, SS, VSS, TKN, NH 3-N, PO4-P, gas composition and for slaughterhouse wastewaters protein concentrations. In particular, results on VFA are presented as concentration, COD of the acids, composition and in relation to the influent and effluent COD. Assessment of the effects of design parameters on the performance of acidogenic biomass are based on: VFA production and composition; acidified COD; and overall effluent quality in relation to methanogenic treatment requirements. This study provides information on all design requirements needed to use acidogenic phenomena to convert organic matter into simple carbon source (i.e. VFA). Such a conversion appears to benefit biological wastewater treatment when used as pre-treatment for anaerobic digestion, but also for its potential in aerobic processes and nutrient removal processes. The process proves to have great low-cost pre-treatment potential, but can also be used for advanced wastewater treatment. Finally, the extensive data collected is used to present various guidelines for process engineers. which should be considered in order to design anaerobic plants. Also, they should be even further used for the overall assessment of the treatment or pre-treatment potential of pre-acidification for agro-industrial wastewaters.