Treatment of high strength dairy industry wastewater and energy production in different anaerobic reactor configurations

The aim of this thesis is the treatment of delactosed whey permeate (DWP) by anaerobic digestion process at mesophilic conditions in two different reactor configurations, specifically semi-continuous stirred tank reactor (s-CSTR) and anaerobic membrane bioreactor (AnMBR), and comparison of methane production potential within systems. DWP is highly polluted dairy wastewater which is the remaining liquid of the lactose production process from the by-product of the cheese-making process, whey. The effect of different organic loading rates on the system response, DWP treatment efficiency, and methane production yields was evaluated in two different reactor configurations (s-CSTR and AnMBR). Moreover, the effect of OLR on membrane fouling mechanism in AnMBR was comprehensively examined. The wastewater DWP used as substrate was provided from the manufacturer, and detailed characterization analyses were carried out. DWP is a moderately acidic liquid with a pH 5,12 and was found to have 22,5% total solids and 14,28% lactose. Total and soluble COD concentrations were determined as 235±3,4 and 213±1,1 g/L, respectively. It was concluded that DWP has a very high pollution load and was a suitable substrate for anaerobic treatment due to its high organic content. Four different wastewater COD concentrations at 2,3,4 and 5 g/L were studied in s-CSTR for a total of 110 days of operation. The increasing wastewater COD concentration decreased COD removal yield, which was determined as 91,1; 90,9; 89,3 and 87,6%, respectively. Additionally, COD accumulation was observed. Methane production yield was obtained as 86,6; 119,3; 89,0 and 58,1 L CH4/gCODremoved, respectively. The highest methane yield was achieved at 3 g/L COD wastewater concentration while a 50% decrease in methane yield was observed at 5 g/L COD wastewater concentration. The high rate of decrease in the methane yield was shown to be due to the inhibition caused by DWP-induced ion accumulation in the semi-continuously operated reactor. It was concluded that the concentration of accumulated ions as Cl- and NH4+ were below inhibition concentration, whereas K+ (6704 ppm, highly accumulated) and Na+ contributed to the inhibition specifically. AnMBR was operated at four different organic loading rates (OLRs) as 1,6, 3,4, 4,3 and 6,8 kg/m3day and constant process parameters (HRT 3 days, Flux 5 LMH and SRT ∞) for a total operation of 169 days. Methane yield was obtained as 299,4; 174,9; 305,2 and 258,8 mLCH4/gCODremoved for the OLRs 1,6, 3,4, 4,3 and 6,8 kg/m3day, respectively. The highest methane yield was achieved at 4,3 OLR, meeting 79% of the theoretical methane yield. The AnMBR system gave an outstanding performance compared to the s-CSTR in terms of methane yield. Unlike s-CSTR, COD removal yield increased with increasing OLR reaching above 98,8% removal yield at every OLR condition tested. To evaluate membrane fouling mechanism in AnMBR; EPS and SMP, CST, particle size distribution analyses were performed on sludge samples. SEM, FTIR, and heavy metal and cation analysis were carried out on used membranes in different OLRs at the end of the operations. Examining the effect of OLR on membrane fouling mechanism in AnMBR revealed that the SMPc and SMPp contents increased with increasing OLR, both of which are crucial factors in the fouling mechanism. Additionally, both CST and normalized CST increased with increasing OLR, which means that the reactor medium became more difficult to filter, and therefore, more frequent membrane clogging may occur. Another fouling indicator, SRF, showed an increasing trend with the increasing OLR, which means forming a tighter and less porous cake layer, leading to an increase in resistance to filtration. Observing more frequent clogging in 4,3 and 6.8 kg/m3day OLR than 1,6 and 3,4 kg/m3day OLR was relatable with increased SMPc, SMPp, CST and SRF parameters during operation. PSD analysis revealed that the average particle size of anaerobic sludge was much larger than the size of membrane pores, so the membrane fouling mechanism was resolved as layer cake formation. 4,3 kg/m3day OLR was determined to have the highest clogging potential with the smallest PSD. SEM images of membranes were examined to reveal that the fouling mechanism was the formation of a dense and non-porous cake layer. FTIR analysis showed that the membrane surfaces were coated with organic pollutants in carbohydrate and protein structures. Heavy metal and cation analysis of the cake layer revealed K+ and Ca+ ions to be the most abundant elements of the inorganic composition.