Destabilisation and inactivation of foam and foam-forming microorganisms in poultry slaughterhouse wastewater
Ethical clearance ref: 2019FEREC-STD-112
Poultry slaughterhouse wastewater (PSW) contains a high concentration of nutrients such as ammonium nitrogen (NH4+-N), phosphorous, particulate matter, proteins, detergents as well as fats, oil, and grease (FOG), and includes a high biochemical oxygen demand (BOD) and chemical oxygen demand (COD). This makes PSW toxic if it is discharged to the environment. FOG leads to fouling of diffusers and membranes in wastewater treatment plants (WWTPs); moreover, it enhances the proliferation of actinomycetes that contain hydrophobic cell walls and produces biosurfactants that result in excessive biofoam formation. For wastewater treatment, conventional activated sludge (AS) systems are used. However, the major drawback of AS systems is the accumulation of biofoam that is associated with the presence of FOG and proteins. To reduce biofoam, the AS systems are periodically dosed with synthetic defoamers; however, these compounds are toxic to the environment, and they are a short-term solution because they only deal with the symptoms and not the cause of biofoaming. As a consequence, this study focused on the production of biodefoamers that destabilizes and inactivate foam-forming microorganisms in PSW treatment systems.
Biodefoamer-producing microorganisms were isolated from PSW, and were assessed for their foam reduction efficiency as a foam collapse rate, under various response surface methodology (RSM) conditions, i.e. (pH 7-10) and biodefoamer concentration (1-4 % v defoamer/ v MLSS and PSW mixture). The isolates that achieved high foam reduction efficiency at a high foam collapse rate were identified and characterised. These isolates were found to be Bacillus subtilis, Aeromonas veronii, Klebsiella grimontii, and Comamonas testosteroni testosteroni, which were then mixed as a consortium and further tested for their foam reduction efficiency and foam collapse rate. At 4% (v defoamer/v MLSS and PSW), the crude biodefoamer had 96% foam reduction efficiency at a 1.7 mm/s foam collapse rate, whereas at 4% (v [ active silicone polymer antifoam A by Sigma-Aldrich synthetic antifoam] / PSW), a synthetic defoamer obtained 96% foam reduction efficiency at 2.5 mm/s foam collapse rate achieved in less than 50s. The use of a biodefoamer resulted in compacted flocs whereas the use of synthetic defoamers resulted in flocs with protruding filaments. Fourier transform infrared spectroscopy (FTIR) showed that the biodefoamer had alkanes, amines, carboxyl, and hydroxyl groups which indicated that the defoamer was a polysaccharide. 1H nuclear magnetic resonance spectroscopy confirmed that the biodefoamer was a carbohydrate polymer.
The mixed liquor of suspended solids (MLSS) that was used to assess biodefoamer efficiency was assessed using metagenomics, and the dominant microorganisms were identified as Nostocoida limicola, Gordonia kroppenstedtii, Candidatus Microthrix parvicella, Nocardioides insulae and Bacteroides nordii, which are biofoamers. Foaming reactors were also designed to assess the effect of reactor design on foamability and foam stability using the mixed liquor of biofoamers. The highest foam stability of 5.1 cm was achieved using a 150 mL sample in a 250 mL foaming rector at 3080 mg/L MLSS concentration, culminating in a foaming potential of 51.3 mL/L. Whereas the lowest foam stability of 0.94 cm was achieved using a 250 mL sample with 4090 mg/L MLSS concentration in a 500 mL foaming reactor, which was the highest concentration of MLSS used in this study. For this setup, a foaming potential of 83.5 mL/L was observed. It was evident that reactor configuration was influential in foamability and foam stability. Furthermore, there was a high concentration of FOG and NH4+-N in PSW, hence it was crucial to assess the effect of these contaminants on the biodefoamer consortium growth as well as the ability of the consortium to biodegrade these contaminants to avoid biofoaming. The highest biodefoamer reduction of NH4+-N from 49 mg/L to 15.8 mg/L was observed at 216 h with FOG decreasing from a concentration of 170 mg/L to 6 mg/L. This indicated that the consortium also produced lipases. To elucidate foam destabilisation in terms of foam drainage and foam collapse rates, biodefoamer kinetics were developed using 3 models, namely the rate law, Monod’s model, and exponential decay formula. The rate law model could predict both foam drainage and collapse, with R2 of 1 and adjacent R2 values of 0.98 respectively. Further, it had low variance of 6.99E-19 and 0.0148 and standard deviation was 2.2 E-10 as well as 0.001. These statistical aspects are a requirement for a good predictive model. Therefore, the rate law was significant in the prediction of the kinetic constants.
The biodefoamer consortium was cultured in PSW and MLSS nutrients to further assess its growth rate in the presence of high FOG and protein concentration as well as foam reduction efficiency. The highest growth rate was observed on day 3, whereby an exponential reduction of FOG from 672 mg/L to 400 mg/L was observed; moreover, proteins were also reduced from 130.8 - 96 mg/L. The foam reduction efficiency of 1.33 mm/s was obtained on day 5, which indicates that the biodefoamers are not only produced during the exponential growth phase but are also produced during cell lysis. The biodefoamer consortium was assessed for its antimicrobial activity by quantifying volumetric zones of inhibition (VZI) on the MLSS and PSW microbial community. The highest growth inhibition was 1.39 L/mL which was observed in the presence of the biodefoamer consortium. The other highest antimicrobial activity of 1.32 L/mL was observed when a biodefoamer produced by Aeromonas veronii and Klebsiella grimontii was used. This revealed that these microorganisms work better when they are in a consortium culture than as monocultures. The consortium was deemed competent for biodefoamer-supported AS systems, but this assertion needed to be evaluated.
Therefore, a miniaturised AS wastewater treatment system which consisted of a primary sedimentation tank, aeration tank, and a secondary clarifier was subsequently designed. The air flow rate in the aeration tank was 7L/min, and a 26-day-old sludge was collected from a local municipal WWTP near Cape Town, South Africa (SA). Subsequently, 1L of the MLSS was added into the aeration thank and PSW was pumped into the reactor using a Gilson® Minipuls Evolution peristaltic pump at a flow rate of 3.4 mL/ min at a hydraulic retention time (HRT) of 24 h. The sludge was recycled from the MLSS recycle steam back into the AS tank using a Gilson® Minipuls Evolution peristaltic pump at a flow rate of 3.2 mL/ min at a sludge retention time (SRT) of 10 days. The biodefoamer consortium was supplemented at 4% (v/v) (biodefoamer: PSW) for the removal of FOG, soluble proteins, total suspended solids (TSS), and COD in the PSW. The results indicated that the biodefoamer-AS aerobic tank can remove up to 94% FOG, 99% soluble proteins, 93.3% TSS, and 85.4% COD; whereas the synthetic defoamer supported AS (Syn-AS) system removed 74% FOG, 79% soluble proteins, 83.2% TSS as well as 61% COD. Furthermore, the conventional activated sludge (CAS) achieved removal of up to 72.3% FOG, 68% soluble proteins, 87% TSS and 50.5% COD. Meanwhile, in the secondary clarifier, the Biodefoamer-AS system removed a further 85% FOG, 99% soluble proteins, 86% TSS, and 67% COD, when compared with Syn-AS which removed FOG, soluble proteins, TSS, and COD by 67%, 92.2%, 86%, and 44.1% respectively. It was therefore concluded that the Biodefoamer-AS system was more efficient in the removal of the contaminants as compared to the Syn-AS and CAS systems. Overall, this study is the first to report on the efficacy of biodefoamer consortium for application in AS systems; however, further development regarding this approach is needed.