Comparison of performance among four different processes for the anaerobic treatment of wastewater with a low concentration of organic matter

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Abstract
As anaerobic treatment of wastewater with a low concentration of organic matter (low‐strength wastewater), an improved conventional process with recycling of sludge (hereafter called sludge‐recycling process) and some novel processes were tested using synthetic wastewater (TOC, 100 to 2000 mg l‐1) at a volumetric loading rate of TOC of 0.8 g l‐1d‐1 and performance of each process was evaluated. The sludge‐recycling process was unsuitable for the treatment of the synthetic wastewater of 2000 mg TOC l‐1, and an ORP (oxidation‐reduction potential)‐control system was even worse. By contrast, the novel processes involving an anaerobic fluidized‐bed reactor (AFBR) an upflow anaerobic sludge blanket (UASB) and an upflow anaerobic filter process (UAFP), which can hold a high concentration of sludge, each process treated low‐strength wastewater efficiently. In particular, AFBR and UASB reached a removal efficiency of TOC of 80% for the lowest‐strength wastewater (TOC of 100 mg l‐1, which corresponded to a BOD of 250 mg l‐1). Under appropriate conditions (volumetric loading rate, 0.8 g TOC l‐1d‐1; concentration of TOC in influent, 100 mg/l) AFBR also maintained a removal efficiency of TOC of 80%, even when the temperature of treatment was lowered to 15 °C. Under these conditions, it was found that the treatment by AFBR could be performed with a very short hydraulic retention time (HRT) of three hours, even if the volume of the support medium in the reactor was neglected in the calculation.