Biosurfactants are often discussed in reference to bioremediation and wastewater treatment. Microbial produced surface tension depressants, biosurfactants, are produced by many microbes to improve efficacy of exocellular enzymes in brining vitals components across the cell wall. The biosurfactants improve access to non-polar organics (fats, oils & grease) while also enhancing access to inorganic micronutrients. The biosurfactants also play a role in the biofilm formation.
Biosurfactants are not a single compound, each group manufactures different biosurfactants that exhibit differing properties. The most commonly encountered biosurfactants are:
Glycolipids - low molecular weight compounds consisting of carbohydrate and lipid components. Synthesized by representatives of the Rhodococcus, Nocardia, Pseudomonas, Candida genera.
Lipopeptides - low molecular weight compounds consisting of a peptide (protein) and lipid parts, the most well-known are from Bacillus genus.
Fatty acids and phospholipids - fatty acids are formed at alkanes oxidation and may be released into the environment by Rhodococcus sp. Phospholipids are synthesized by representatives of the Acinetobacter and Corinebacterium genera.
Polysaccharides - extracellular polymeric compounds with powerful emulsifying properties. Such biosurfactants formed by a wide range of microbes and are part of the biofilm formation process.
All biosurfactants can be divided into two major groups - low (glycolipids, lipopeptides) and high (extracellular polymers) molecular weight surfactants, which differ in physiology and surface active properties.
Biosurfactants are interesting to scientists as they are highly effective at reducing water surface tension at low concentrations. Additionally, biosurfactants exhibit superior properties to synthetic surfactants including:
· Lower toxicity
· High biodegradability
· Activity over a wide range of pH and salinity values
· Ability of biosurfactants to be manufactured in-situ or from waste compounds in industrial production.
I had a request to see if a low cost biological pretreatment system could lower POTW surcharges and meet permit requirements for an expanding facility that dewaters portable toilet wastes. This waste has a high organic concentration with an influent COD 6,000 mg/L. Additional concerns include hydrogen sulfide and other odor causing compounds.
The facility has limited space for a biological treatment unit and does not want to incur extensive capital expenditure. A initial bench study was done over a 10 hour period to determine if biological treatment could reduce COD.
First the bench test was done using bioaugmentation cultures designed for high COD wastewater – the microbial seed included organisms selected from Bacillus, Pseudomonas, and Rhodococcus species. Preserved in a dry form, we decided to not pre-acclimate or revive the product before adding to the inlet sample.
Temperature was approximately 23 Deg C during the test. Aeration and mixing were provided with a normal aquarium airstone & pump.
We pulled samples at time 0, 6, & 10. This is because the maximum HRT we can realistically have on the site is 12 hours.
One issue is that it takes at least 4 hours for the cultures to “revive” and acclimate to the waste. Therefore, kinetics from this run are not suitable for eventual prediction of how well the system will run. In a working system, there is always an active biological portion that is kept after discharge when a pretreatment system is operated in batch mode. The expected design is an aerated tank (possibility of adding fixed media will be considered), with additions of bioaugmentation cultures during periods of high flow, difficult temperatures, and when loadings have variation.
The initial results from COD tests are presented in the figure below. During the 10 hour test COD decreased from 5,900 mg/L to 3,400 mg/L.
While I ran a regression on COD reduction versus time, the only reliable conclusion from this test is that the waste is treatable using aerobic biological pretreatment. A second set can be done with an acclimated biomass (give at least 48 hours on aerated waste influent). From this 48 hour aerated flask, discharge 75% of the waste and recharge with fresh influent. Take samples at time 0, 4, 8, and 12 hours.
Many industrial discharges have high levels of chlorides which have multiple impacts on the microbes vital for wastewater treatment. The main impacts are related to cells maintaining cell wall integrity (maintaining turgor pressure), extracellular polysaccharides formation, and interference with enzyme activity.
Most common waste water bacteria function well from low Total Dissolved Solids and/or chlorides up to 3.5% (or near the concentration of salinity in the ocean). Once you go above this point, the microbial population makeup begins to change to favor more halotolerant microbes. This region like similar inflection points in temperature and pH can be difficult to develop a stable biomass. Often our commonly studied wastewater microbes cease growth around 4.5% chlorides and switch to halotolerant strains is mandatory.
At Aster Bio, I have been working with wastewaters having chlorides up to 5% even after chloride reduction. Additionally, microbial enhanced oil recovery often has oil reservoirs with chlorides above 7 - 10%. In effort to address these difficult wastewaters, we have been evaluating a number of halotolerant strains that thrive above 4% chlorides.
Interestingly, we have found some halotolerant strains isolated from desert environments that have evolved a resistance to chlorides and other common dissolved ions that can stress many common organisms. Current screening has shown excellent growth at various chloride concentrations.
From the research on these strains, I have developed a chart on chloride tolerance of various microbes in Aster Bio's catalog:
Erik Rumbaugh has been involved in biological waste treatment for over 20 years. He has worked with industrial and municipal wastewater facilities to ensure optimal performance of their treatment systems. He is a founder of Aster Bio (www.asterbio.com) specializing in biological waste treatment.
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