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There is considerable interest in recuperative thickening (RT), the recycling of partially digested solids in an anaerobic digester outlet stream back into the incoming feed, as a 'high-performance' process to increase biogas production, increase system capacity, and improve biosolids stabilization. While polymer flocculation is commonly used in full-scale RT operations, no studies have investigated the effect of flocculation conditions on RT process performance. Our goal was to investigate the effect of polymer type and dosage conditions on dewatering performance and biogas production in a lab-scale RT system. The type of polymer flocculant significantly affected dewatering performance. For example, the 440 LH polymer (low molecular weight (MW) polyacrylamide) demonstrated lower capillary suction time (CST) and filtrate total suspended solids (TSS) values than the C-6267 polymer (high MW polyacrylamide). An examination of the dewatering performance of RT digesters with different polymers found a strong correlation between CST and filtrate TSS. The type of polymer flocculant had no significant effect on biogas productivity or composition; the methane content was greater than 60% in good agreement with typical results. The optimization of the polymer flocculation conditions is a critical task for which the lab-scale RT system used in this work is ideally suited.

In this work, a new microscale flocculation test (MFT) method was developed that is ideally-suited for optimizing separation performance. A critical and complicated task in wastewater treatment is to identify the flocculation conditions that yield the optimal separation of water from suspended solid materials. The standard 'jar test' method is inadequate for conducting a full process optimization because a typical set-up only allows for a maximum of 6 tests to be conducted at once, and fairly large volumes of materials (approximately 1 to 2 litres) are needed for each individual test. The microplate-based, parallel processing format of the MFT method allows for many dozens of flocculation tests to be conducted simultaneously, with each test requiring only a few millilitres of material. As a demonstration of the MFT method, ten cationic polymer flocculants were evaluated with various digestate types. The optimal separation performance, as determined by the lowest capillary suction time (CST) measurement, was found by rigorously evaluating the effect of flocculant type (including molecular weight and charge density) and dosage conditions (including total amount added and single versus staged addition).For example, the dose-dependent profiles for certain flocculants exhibited a nearly 10-fold greater decrease in CST compared to other flocculants. Process optimization in environmental separations is not trivial, but rather, is a complicated task that requires an extensive amount of experimental work for which the MFT method is ideally suited. For any dewatering process, an ongoing challenge involves selecting, from the vast number of possible flocculants, the 'best' one in terms of economics and performance. In our previous work, we found that the dewatering performance of twenty-four different flocculants from three suppliers varied considerably [4]. Notably, for anaerobic wastewater treatment, the cost of the polymer flocculant is the greatest component in the biosolids treatment operation [5].This leads to two important considerations: first, it is generally understood that there is no workable algorithm to relate flocculant properties to dewatering performance [3]. Secondly, wastewater sources from different treatment plants have diverse dewatering characteristics. So, the exact flocculant type and the optimum dose must be determined on a case-by-case basis for different treatment applications. Yet, the selection of the 'best' flocculant has been described as "one of the most demanding, frustrating, and time-consuming experiences for many operations personnel" [6].The benefit of optimizing the flocculant dose extends beyond the operating costs for a given treatment facility. Water-soluble polymers have been found to be toxic to aquatic lifeforms, even at low concentrations [7]. Thus, there are valid concerns about the effect of flocculant 'overdosing' in dewatering processes, given the potential for discharge into the aquatic environment, either directly via the liquid fraction, or indirectly via leachin...