Characterisation of food service establishment wastewater and its implication for treatment

Abstract

Essential for the selection of a reliable treatment system is the characterisation of the effluent to treat. Kitchen wastewater (KWW) from food service establishments (FSEs) is a strong organic and fat-rich effluent whose characterisation has not been sufficiently addressed. KWW composition is highly variable and linked to the FSE's size, the type of meals prepared and the amount of water used during the cleaning. COD, TSS and fat content (FOG) are the most common parameters found in literature. However, other physical and chemical parameters (e.g. temperature, pH, oil droplets characteristics and trace elements), correlated to commercial kitchen cleaning practices rather than the specific effluent, but equally influential on the treatment efficiencies of both physical and biological methods, have hardly been investigated. A comprehensive characterisation of wastewaters from three food service establishments was used to generate data to support the selection of appropriate FOG mitigation methods. Two novel analytical methods were used to quantify the proportion of emulsified FOG and associated droplet size from different kitchen washing effluents. The results showed that more than 90% of the FOG from the dishwasher effluent and around 35% of sink one was emulsified, with droplet sizes less than 100 µm, well below the removal capabilities of conventional grease interceptors, but easily removed using biological means. From the WW composition results, a formula for predictive modelling was derived to represent average organic matter composition for kitchen wastewater as C20H38O10N, applicable in remediation processes. These results offer a good starting point for the design, operation, and optimisation of wastewater treatment systems of oil-rich KWW.

Understanding why fat, oil and grease (FOG) bioremediation can be unsuccessful

“…A basal mineral medium was designed using a kitchen wastewater characterisation previously reported (Gurd et al 2019). The synthetic KWW composition (recipe reported in table 1S) contained calcium chloride dihydrate (100 mg/l), potassium chloride (70 mg/l), magnesium sulphate heptahydrate (40 mg/l), ammonium chloride (10 mg/l), iron chloride (0.27 mg/l), zinc sulphate heptahydrate (0.16 mg/l), copper sulphate (0.07 mg/l), manganese sulphate monohydrate (0.045 mg/l), cobalt nitrate hexahydrate (0.002 mg/l) and ammonium heptamolybdate tetrahydrate (0.001 mg/l), together with a non-ionic surfactant Triton X-100 (14 mg/l).…”

“…Filter-sterilised rapeseed vegetable oil (Tesco, UK) was added dropwise to the flasks with a sterile pipette. Concentrations of the organic components were varied to achieve the test parameters reported in Table 1 covering ranges previously determined in real KWW (Gurd et al 2019).…”

“…Table 1. Test conditions for the four experimental trials showing concentrations of individual substrates andCOD(theoretical):N ratios, compared to concentration ranges in real kitchen wastewater (Gurd et al 2019).…”

“…FOG removal rates were measured in batch tests containing different initial oil concentration (150, 300, 450, 600 and 800 mg/l) which reflected the range observed in real KWW (Gurd et al 2019). Log phase bacterial growth was observed during the first 8 hours in association with rapid depletion of soluble carbohydrates, whilst, except for the lower FOG concentration (150 mg/l), minimal FOG removal was observed (Figure 1A),compared to 95 ± 4.4% extraction efficiency in control samples.…”

“…This ratio provides enough carbon to allow all available nitrogen to be co-synthesised into new biomass together with just the right amount of carbon energy to drive the process. Gurd et al (2019) reported ratios of 60:1 and 50:1 for sink and mixed effluents wastewater and 75:1 for dishwasher discharges, suggesting a significant N deficiency for successful biological treatment leaving excess organic carbon undegraded. Nevertheless, microbial populations have the ability to adjust their metabolic processes in response to nutrient deficiency/carbon excess, whereby the balance between synthesis and energy shifts towards surplus energy production under carbon excess conditions (Dauner et al, 2001).…”