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A review of the theoretical basis for the design and operation of fluidized bed reactors for anaerobic treatment of wastewaters is presented. After the description of the characteristics of three-phase fluidized bed and theoretical aspects which could affect the operation of an anaerobic reactor, the design considerations and recommendations based upon characteristics of the wastewater to be treated are presented. A step by step design strategy, including media selection, operative conditions of the reactor, equipment sizing and long-term run effects, as well as mechanical design hints on distribution system, reactor body, piping of recycle lines and gas collection are included.

A sequential extraction procedure was applied to two anaerobic methanogenic sludges (Eerbeek and Nedalco) to examine the speciation of micro- and macronutrients in the sludges after cobalt sorption by exposing the sludge to a 1 mM Co solution for 4 d at pH 7 and 30 degrees C. The effect of different physicochemical conditions on cobalt sorption was studied as well: effect of pH (6-8), effect of competition by a second trace element (Ni or Fe), modification of the granular matrix by glutaraldehyde or heat treatment, and EDTA (ethylenediaminetetraacetic acid) addition. Sorbed Co was found to distribute between the carbonates, organic matter + sulfides, and residual fractions. Cobalt adsorption resulted in an antagonistic interaction with other metals present in the granular matrix, evidenced by the solubilization of other trace elements (e.g., Ni, Cu, and Zn) as well as macronutrients (especially Ca and Fe). Modification of the sludge matrix by glutaraldehyde or heat treatment, or exposure to EDTA, led to serious modifications of the Co sorption capacity and strong interactions with multivalent cations (i.e., Ca(2+) and Fe(2+)).

This paper presents a summary of the ideas expounded in the presentations and the resulting discussions at the workshop held in Valladolid, Spain (24-26 September 1990). The formal presentations and discussions are included in the articles which comprise the rest of this issue. The main topics summarized are the characteristics of the anaerobic process, its technological implications relating to different reactor concepts and to different designs, the importance of correct waste characterization for appropriate treatment, the characterization of the biomass, and the influence of all these factors on the design and operation of anaerobic treatment systems. Sections outlining the research and development needs for the enhancement of this technology are also included.

The effect of the addition of trace elements on the conversion of a mixture of volatile fatty adds (Acetate, Propionate, Butyrate, in a ratio 3:1:1) by anaerobic granular sludge was investiated. Two Upflow Anaerobic Sludge Bed reactors (pH 7, ranging 30 degrees C) were operated for 140 days at an organic loading rate from 2 g COD l(-1) d(-1) up to 10 g COD l(-1) d(-1) and a hydraulic retention time of 12 hours. One reactor (R1) was supplied with a trace metal cocktail in the basal medium, whereas trace metals were omitted from the influent of the second reactor (R2). As a result, the trace metal concentration in the granules from R2 steadily decreased at a rate of 48 microg metal g(-1) TS d(-1) down to 35% of their initial value. In contrast, trace metals accumulated in granules present in R1. At the end of the experiment, the COD removal efficiencies were 99 and 77% for, respectively, the control (R1) and deprived (R2) reactors. This difference was due to lack of propionate conversion by sludge from R2. No difference in the acetate and butyrate conversion capacity of both reactors was observed. The conversion of acetate, propionate and methanol were stimulated by the continuous addition of metals to the influent, a sludge of R1 had higher maximum specific activity values compared to sludge of R2. However, both sludges had a similar maximum specific activity with butyrate. Surprisingly, maximum specific activity tests using individual trace metals showed that the addition of a particular trace element in the activity test medium did not affect the degradation rates of aspecific substrate, i.e. acetate, propionate, butyrate and methanol.