JOURNAL OF FERMENTATIONAND BIOENGINEERING VO1.77, No. 6, 679-686. 1994 Treatment of Low Strength Soluble Wastewaters in UASB Reactors MARIO T. KATO,* JIM A. FIELD, ROBBERT KLEEREBEZEM, AND GATZE LETTINGA Department of Environmental Technology, Wageningen Agricultural University, Biotechnion, Bomenweg 2, 6703 HD IYageningen, The Netherlands Received 6 December 1993/Accepted 18 March 1994 Low strength wastewaters can be those with chemical oxygen demand (COD) below 2,000 mg/l. The anaero- bic treatment of such wastewaters has not been fully explored so far. The suboptimal reaction rates with low substrate concentrations, and the presence of dissolved oxygen in the influent are regarded as possible con- straints. In this study, the treatment of low strength soluble wastewaters containing ethanol or whey was studied in lab-scale upflow anaerobic sludged bed (UASB) reactors at 30°C. The high treatment performance obtained demonstrates that UASB reactors are viable for treating both types of wastewaters at low COD con- centrations. The treatment of the ethanol containing wastewater resulted in COD removal efficiencies ex- ceeding 95% at organic loading rates (OLR) between 0.3 to 6.8 g COD/I. d with influent concentrations in the range of 422 to 943 mg COD//. In the case of the more complex whey containing wastewater, COD removal efficiencies exceeded 86% at OLRs up to 3.9 g COD/I.d, as long as the COD influent was above 630 mg/l. Lowering the COD influent resulted in decreased efficiency with sharper decrease at values below 200 mg/l. Acidification instead of methanogenesis was found to be the rate limiting step in the COD removal at low concentrations, which was not the case when treating ethanol. The effect of dissolved oxygen in the influent as a potential danger in anaerobic treatment was investigated in reactors fed with and without dissolved oxygen. Compared with the control reactor, the reactor receiving oxygen showed no detrimental effects in the treatment performance. Thus, the presence of dissolved oxygen in dilute wastewaters is expected to be of minor importance in practice. The upflow anaerobic sludge blanket (UASB) concept is a simple design which has been demonstrated as a very efficient process for wastewater treatment. The success of the UASB can be attributed to the adequate retention of dense granular sludge with high specific activity, ena- bling good treatment performance at high organic load- ing rates (OLR) (1-3). Mixing of the bulk liquid phase, which is considered essential for optimal wastewater-bio- mass contact, is obtained in the UASB reactor by the natural turbulence. This turbulence is firstly achieved due to the high biogas production at high OLR and secondly due to the hydraulic load resulting from the influent flow. An even influent distribution system is also required (1, 4). The UASB was originally developed for the treatment of industrial wastewaters of medium to high strength. However, the potentials of the UASB is not necessar- ily restricted to medium and high strength wastewaters. Its application is being explored more and more for the less concentrated wastewaters with chemical oxygen demand (COD) below 2,000mg/l (5; Maaskant, W. & Zeevalkink, J.A., Proc. Eur. Symp. Anaerob. Waste Wat. Treatment, The Netherlands, p. 430, 1983; Pfeiffer, W. et al., Proc. Wat. Treatment Conf., p. 219, 1986). Attempts have been made towards the treatment of low strength domestic sewage with full-scale UASB plants. Several research groups are reporting satisfactory per- formance at temperatures above 20°C (2, 6-8; Vieira, S. M. M. & Garcia, A.D. Jr., Proc. Sixth Int. Symp. Anaerob. Digest., Brazil, p. 133, 1991; Draaijer, H. et al., Proc. Sixth Int. Symp. Anaerob. Digest., Brazil, p. 115, 1991; Lettinga, G. et al., Proc. 2nd Int. Symp. Anaerob. Digest., Germany, p271, 1982; Schellinkhout, * Corresponding author. A. & Collazos, C.J., Proc. Sixth Int. Syrup. Anaerob. Digest., Brazil, p. 145, 1991). Treatment performance efficiencies of 70% to 89% were achieved at influent con- centrations in the range of 267 to 660mg COD/l, at OLRs up to 3.8g COD~I.d. Nonetheless, the use of UASB for treating low strength soluble industrial wastewaters has not yet been fully explored. Research with other high rate systems like the anaerobic attached expanded bed (AAFEB) or fluidized bed (FB) in lab- and bench-scale reactors has been conducted. Researchers claim that these system may be more effective for low strength wastewaters due to the superior mass transfer at high hydraulic loads in these reactor types compared with that of the UASB (9, 10; Lettinga, G. & Vinken, J.N., Proc. 35th Indust. Waste Conf., Purdue Univ., p. 625, 1980; Switzenbaum, M. S. & Jewell, W. J., Proc. 51st Annu. WPCF Conf., USA, p. 1, 1978; Maragno, Ao L. F.C. & Campos, J.R., Proc. Sixth Int. Symp. Anaerob. Digest., Brazil, p. 157, 1991). Nonetheless, some investigations with the UASB indicate that its appli- cation can be valid as well for low strength wastewaters. When treating synthetic wastewater consisting of cheese whey, a COD removal efficiency of up to 89% was obtained at OLR of 2.6g COD/I.d and at influent COD concentration (CODin) of 500mg/l (Chernicharo, C. A. L., Ph.D. thesis, Univ. of Newcastle Upon Tyne, England, 1990). The treatment of paper recycle industry wastewater of less than 1,000 mg COD/I in a full-scale UASB reactor resulted in an efficiency higher than 70% at OLRs ranging from 4.4 to 5.0g COD/I.d (5). A number of factors can be enumerated that could ex- plain the limited application of the UASB reactor for low strength wastewaters. The low concentration and the possible presence of dissolved oxygen in some dilute wastewaters are possibly the main constraints. The low 679