Treatment of high strength and toxic organic industrial effluents in the anaerobic baffled reactor (2001)

The predicted high industrial growth rate in South Africa, coupled with the limited water resources, will necessitate the implementation of advanced effluent treatment systems. The results of previous research identified a number of industries, in KwaZulu-Natal, that produced effluent streams that would be amenable to treatment by anaerobic digestion. The organic content of these effluents (based on COD) was too high to permit conventional treatment at a wastewater treatment works. In the short term, these effluents could be treated in the under-utilised anaerobic digestion capacity in the region. However, with the projected increase in the load on wastewater treatment works, this available capacity will ultimately be needed for sewage treatment.

The challenge in advancing anaerobic digestion for high-strength or toxic organic waste streams lies largely in enhancing the bacterial activity taking place per unit of reactor volume and, in the case of xenobiotics (substances having structural features that are not normally encountered in nature), in the acclimation of the biomass to the compound. Taking into consideration the slow growth rate of many anaerobic microorganisms, particularly methanogens, the main objectives of efficient reactor design must be high retention time of bacterial cells within the reactor, together with good mixing to ensure contact between cells and their substrate. The anaerobic baffled reactor (ABR) achieves both objectives by means of a design that is both simple and inexpensive to construct, since there are no moving parts or mechanical mixing devices. High rates of hydraulic throughput are possible with very little loss of bacteria from the reactor.

The ABR has alternately hanging and standing baffles (Fig. 1.1), which divide it into 8 compartments. The lower edges of the hanging baffles are slanted (45 °) to route the flow of liquid and to reduce channelling. The liquid flow is alternately upward and downward between the partitions. The downflow chamber is narrower than the upflow chamber to prevent accumulation of biomass in the latter. On its upward passage, the waste flows through an anaerobic sludge blanket. Hence, the waste is in contact with the active biomass but, because of the design, most of the biomass is retained within the reactor. In principle, all phases of the anaerobic degradation process can proceed simultaneously. The sludge in each compartment will differ depending on the specific environmental conditions prevailing and the compounds or intermediates to be degraded. A staged reactor can provide higher treatment efficiency since non-labile substrates will be in an environment conducive to degradation. Process stability is good.

The concentrated, variable and intermittent nature of industrial effluents make them intrinsically unsuitable for treatment in a completely mixed system. The ABR provides resilience to intermittent organic and hydraulic loads as well as toxic components of an effluent. Cleaner production is defined as the continuous application of an integrated environmental strategy, applied to processes, products and services to increase eco-efficiency and to reduce risks for humans and the environment. Implementation of cleaner production and waste minimisation practices, at the effluent source, will lead to the production of more concentrated effluents. Agro-industries characteristically produce effluents of a xenobiotic nature and/or of high organic strength. Industries such as the food, leather and textile industries also utilise synthetic colorants in their processes, resulting in coloured effluents. 

Dyes and pigments are usually released into the environment in the form of a dispersion or a true solution in the industrial effluent. The presence of very small amounts of dyes in water (less than 1 mg/L) is highly visible and aesthetically unpleasant [Cooper, 1995 #259]. Public perception of water quality is influenced by colour; unnatural colour is associated with contamination. Strong colours could reduce light penetration, thus affecting the growth of plants and the aquatic ecosystem. The current environmental concern with azo dyes revolves around the potential carcinogenic health risk that they present or their intermediate biodegradation products when exposed to microflora in the human digestive tract. There is the potential for the degradation products of these dyes to build up in the environment since many of them pass through wastewater treatment plants virtually untreated. Azo dyes are intentionally designed to be recalcitrant under typical usage conditions, and it is this property, allied with their toxicity to microorganisms, that makes biological treatment difficult.

In South Africa, the ABR could be implemented on-site for pre-treatment of agro-industrial wastes, with high COD contents or those with xenobiotic components, which prevent conventional treatment at a wastewater treatment works. Current disposal options include marine discharge, co-disposal on municipal landfills or dilution with potable water. With implementation of the ABR, waste minimisation practices could be adopted to concentrate effluents on-site. Pre-treatment in the ABR, with a biomass acclimated to the particular effluent, should facilitate sufficient degradation such that the effluent could then be discharged to sewer for treatment.