Distillery Stillage Water Pollution Control: An Integrated Approach for Efficient Treatment and Recovery
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Date
2021-05-26
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Addis Ababa University
Abstract
In spite of the increasing demand for potable and fuel ethanol, the distillery industries continue tobe a major source of surface water and soil pollution, especially to developing nations, due to therelease of a highly organic and complex byproduct- the stillage. Stillage contains solids,recalcitrant organics, persistent color, sulfate, chlorides, phosphorus and nitrogen, whose recoveryand treatment plays a major role in environmental sustainability. In most cases, the application ofsingle stillage treatment technology did not succeed. The objective of this study was to quantifymolasses and stillage byproducts in Ethiopia and examine the feasibility of an integrated treatmentapproach towards sustainability. Standard methods were applied, beginning from a systematicliterature review through primary data collection, stillage sampling and conducting a series ofexperiments. The trials were set-up based on a sound design followed by standard analytics and astatistical analysis of the results. The experiments started with determining the biochemicalmethane potential (BMP) test and then undertaking a mild iron oxide coated sand (IOCS) basedwet air pretreatment (WAp) through anaerobic digestion (AD), adsorption and a polishing aerobicdegradation.From a quantitative characterization study, Ethiopia currently produces close to 1.8 million tonsof cane sugar, with the release of over 300,000 tons of molasses as a by-product. Using canemolasses as a raw material and with the release of 431,000 m3 of stillage into the environment, thepotable ethanol industry in the country produces over 33,000 m3 of ethanol annually. Accordingto the BMP test run for 45 days at mesophilic condition, molasses stillage has the potential ofgiving 139 NmlCH4/g-COD with 68% purity. A mild IOCS based WAp of the stillage sample at3.5% IOCS loading, a temperature of 60 oC, which was held for four hours at atmospheric pressurebrought the desirable effect in detoxification. The result observed following comparison of rawand treated stillage from E. coli incubation at 37 oC and 86 revolutions per minute of shakingdemonstrated a statistically significant difference (p-value = 0.02) in toxicity. Successively, theconsequence of a mild, IOCS based WAp on the methane (biogas) yield as well as the biochemicaloxygen demand (BOD) and COD removal was compared with the raw counterpart using batchmesophilic (35±2 oC) AD of stillage. Further, the effect of applying scoria (a highly vesicularigneous rock) support on AD process stability and performance was assessed for the first time.Consequently, a statistically significant (p-value < 0.05) difference in the cumulative specific raw withoutascoria support, raw with scoria support and WAOp scoria supported stillage AD subsequently. Thesoluble COD (CODsol) after AD of stillage with scoria, without scoria and with scoria and WApin mg/l was 30041.7, 30666.7, and 13375.0 respectively. With the WAOp and scoria supportedstillage AD a BOD and COD reduction of ≈100% and 92% were achieved. These achievementsare unique in stillage recovery and pollution remediation findings so far. Further, the biogas yieldwith respect to the reaction days (within 18 days) with a relatively stable process (average pH=7.7,6.8 and 6.7 for pretreated and untreated stillage with scoria and without scoria) was hastened dueto pre-treatment.Regarding the adsorption trial on stillage after the batch AD, all the significant COD removalswere observed after three hours of contact time for all doses of the treated scoria adsorbent.Maximum removal rate was at 0.03 mg/ml of scoria dosing and in three hours contact timewhereby the final COD of the stillage did go lower only to 7500 mg/l.In a related experiment, the effect of organic loading rate (OLR) and stillage WAp was studied ina continuous AD in scoria-packed column reactors. The removal of stillage COD was significantlybetter (p-value = 0.036), with an average of 13% difference, due to pre-treatment of stillage withan increased OLR to 2000 mg/L-d. In a further polishing aerobic treatment trial of the effluentfrom the stillage batch AD, a complete removal of the BOD and significant removal of COD, withan average effluent COD of 2278 mg/L was achieved. In fact, 68% of the COD removal occurredwithin eight hours of digestion. Despite the persistence of color, the removal of organics withbioenergy recovery from integrating the WAp, the batch AD and the aerobic remediations (˜99%COD and 100% BOD removals) of stillage appear to be a promising technique in its sustainablemanagement while adhering to the regulators’ discharge limits.
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Keywords
Distillery Stillage Water Pollution Control Integrated Approach Efficient Treatment Recovery