The influence of excess sludge discharge on the performance of a full-scale UASB reactor K. K. Barros a,b , S. Gavazza a, *, L. Florencio b and M. T. Kato b a Laboratory of Environmental Engineering, Academic Center of Agreste, Federal University of Pernambuco, Rodovia BR-104, Km 62, Nova Caruaru. Caruaru, Pernambuco CEP: 55002-970, Brazil b Laboratory of Environmental Sanitation, Department of Civil Engineering, Federal University of Pernambuco, Av. Acadêmico Hélio Ramos, s/n. Cidade Universitária, Recife, Pernambuco CEP: 50740-530, Brazil *Corresponding author. E-mail: savia@ufpe.br Abstract The influence of sludge discharge frequency on the performance of a full-scale UASB-type reactor has been assessed. The reactor has eight compartments and treats domestic sewage with an 8 hour hydraulic retention time. In each compartment, a different discharge frequency was employed ranging from 1 to 8 months. During the assessment period, the chemical oxygen demand (COD) removal efficiency, effluent volatile suspended solids (VSS) concentration, and cellular yield coefficient (Y) were evaluated. The results indicated that the operational stability increased with increasing discharge frequency, as shown by a higher COD removal and lower effluent VSS. The values of Y decreased when the sludge discharge frequency increased. In practice, good performance can be achieved when the sludge discharge frequency is between 1 and 4 months; COD removal efficiencies of 60–80%, average VSS effluent concentrations of 25–80 mg L À1 , and Y values of 0.10–0.18 kg VSS kg COD removed À1 were obtained. Key words: cellular production, COD removal, domestic sewage, effluent VSS concentration INTRODUCTION Anaerobic digestion (AD) was previously considered to treat only high-strength wastewaters at temp- eratures above 20–25 °C (Bergamo et al. 2009). Climatic conditions in countries like Brazil are favorable for AD and this technology has been used to treat low-strength wastewaters, such as dom- estic sewage, since the 1980s. AD became more widespread with the introduction of upflow anaerobic sludge blanket (UASB) reactors (Lettinga et al. 1980). At present, anaerobic treatment of wastewaters is widely accepted as reliable and is used extensively in practice. However, it is known that compared to an aerobic system, anaerobic processes have slow growth rates, mainly associated with methanogenic archaea. So, long solids retention times are required because only a small portion of the degradable organic waste is synthesized into new cells. Inside reactors, a dense sludge bed is established at the bottom, and the majority of the biological transform- ations in it are effected by bacteria in aggregates of flocs and granules. These aggregates have good settling properties and are not susceptible to system washout for practical reactor conditions (Veeresh et al. 2005; Latif et al. 2011). UASB reactors also play an important role in wastewater treatment in tropical countries because of the favorable temperature conditions (Kato et al. 2003; von Sperling et al. 2005; Chernicharo 2006). Since about 1985, analytical and computational developments have increased understanding of the parameters and operating conditions that influence anaerobic reactor performance. Nevertheless, © IWA Publishing 2015 Water Practice & Technology Vol 10 No 2 250 doi: 10.2166/wpt.2015.027 Downloaded from https://iwaponline.com/wpt/article-pdf/10/2/250/88704/wpt0100250.pdf by guest on 23 May 2020