The Application of Crossflow Membrane Filtration to Remediate Wheat Starch Processing Wastewater for Reuse

Abstract

This research project investigated the practical application of crossflow membrane filtration technology to treat wheat starch waste streams at an operating wheat starch manufacturing plant, where costs and compliance issues had a negative impact on company business. Water usage, wastewater generation, wastewater treatment and costs were investigated and membrane filtration trials undertaken. These data were utilised to determine system requirements and enable project costs to be calculated and investment alternatives compared. Water usage at the site averaged 530 kL/day (standard deviation = 194 kL/d) to 600 kL/d (standard deviation = 89 kL/d) with 8.7 kL to 9.0 kL water used per tonne flour throughput. Waste streams discharged to sewer and Liquid Fertiliser represented around 70-75% and 20% of incoming water. Three primary waste streams from the factory were combined and treated through the on-site wastewater treatment plant (WWTP). The WWTP volume was 340 m³ to 370 m³ and biogas produced off-set natural gas costs. The treatment train consisted of a head tank, preacidification tank, mix tank, anaerobic digester and aeration tank. Although designed to treat a hydraulic loading rate (HLR) of 13 m³/h and an organic loading rate (OLR) of 4.4 t/d the WWTP was regularly overloaded treating up to 26 m³/h HLR and 17.3 t/d OLR and not always operated within optimal conditions. Freshwater inputs were $1.13/kL and wastewater sewerage discharge costs were based on volume ($0.39/kL) and quality parameters such as biochemical oxygen demand measured over a five day period (BOD5) ($1.07/kg) and suspended solids (SS) ($0.42/kg). Water input costs were in the vicinity of $220,000/y and wastewater discharge costs ranged from $520,000/y to $3.2 M/y. Other disposal costs included anaerobic sludge ($169,000/y) and Liquid Fertiliser ($263,000/y). Ceramic membranes, manufactured by Fairey Industrial Ceramics, UK with different pore sizes (0.2 μm, 0.35 μm and 0.5 μm) were trialled with untreated wheat starch waste streams. Total solids, SS and chemical oxygen demand (COD) concentrations were reduced by up to 30%, 88% and 92%. Cleaning efficiencies of up to 70% were achieved with sodium hydroxide solutions. No benefits were gained by increasing transmembrane pressure (TMP) above 100 kPa. Backflushing 30 seconds every five minutes increased permeate flux. Permeate flux frequently increased over time using The application of crossflow membrane filtration technology to remediate wheat starch processing wastewater for reuse wheat starch wastewater as the feed, unlike when model starch solutions were used. Concentrated solids had a greater value as a stockfood component (average $546/d) than as biogas (average $227/d). Anaerobic effluents were treated using two different membrane types simulating sidestream MBR and submerged MBR processes. Using the side-stream MBR, effluent was progressively filtered in a multi-stage microfiltration and ultrafiltration process. Flux ranged from 51-60 L/m²⋅h and COD reduction was 84%-87%. Total nitrogen (TN) and total phosphorus (TP) were completely removed (0.1 μm membrane) and metal concentrations decreased for calcium, potassium, magnesium, aluminium, copper, iron, manganese, zinc and cadmium. Cleaning efficiencies were least for the 0.1 μm membrane and increased as membrane pore size decreased. Anaerobic permeate samples using a 0.4 μm submerged membrane contained 1.5 mg/L TP (original sample 61.9 mg/L) and 13.3 mg/L TN (original sample 350 mg/L). Bioavailable nitrogen compounds were 3.9 mg/L and 0.043 mg/L for N-Ammonia and N-Oxides respectively. Filterable reactive phosphorus concentration was 0.23 mg/L. Anaerobic permeate quality was suitable for crops moderately tolerant to salinity (0.1 μm and 0.4 μm membranes), could cause foliar injury to plants and posed a low risk of complexing with cadmium in soil. TP and TN levels were suitable for irrigation use long term (100 years) (0.1 μm membrane) and short term (20 years) (0.4 μm membrane). Increased biogas generation, as a result of COD being retained by a side-stream 0.1 μm membrane, could provide additional energy (43,000 GJ/quarter) with a natural gas equivalent value of around $500,000. Financial analyses were undertaken on four investment alternatives and the application involving microfiltration of untreated wastewater was identified as the least cost option for managing financial and other issues associated with wastewater discharge at the wheat starch factory.