Abstract
The process of cassava starch extraction is associated with the generation of a large volume of liquid wastes from root washing and starch extraction (5 to 7 L.kg-1root). In this context, the objective of this study was to evaluate the treatment of cassava starch wastewater in a horizontal anaerobic fixed bed reactor to the removal of organic matter and generation of biogas. For this purpose, two fixed bed reactors filled with two different types of support materials were used: bamboo rings and flexible PVC rings. The reactors were constructed in polyvinyl chloride (PVC) with 90 cm length and 15 cm diameter. The working volume of R1 (bamboo as support material) and R2 (PVC as support material) were 6.0 and 8.0 L, respectively. In the operation of the reactors, 13 assays (A1 to A13) were carried out with an increase in the organic loading rate (OLR: 1.7 to 15.0 g COD.L-1.d-1) and a decrease in the hydraulic retention time (HRT: 4.0 to 0.8 days). Biogas production followed a similar pattern in both reactors. The highest biogas productions of 1.5 L.L-1.d-1 (R1) and 1.1 L.L-1.d-1 (R2) were verified in the assay 6 in which were applied highest affluent concentration (15,122 mg.L-1), intermediates organic loading rate (5.6 g.L-1.d-1) and hydraulic retention time (2.7 d). The removal of COD increased with the increase of OLR resulting in COD removal values of up to 99%. Total solids removal efficiencies of 79 and 87% were achieved in R1 and R2, respectively. In both reactors, the contact surface of bamboo (132 m2.m-3) and PVC (191 m2.m-3) supported biomass attachment even in highest OLR (15 g.L-1.d-1) and low HRT (0.8 d). The both support material provided conditions to the reactors withstand the variations and reestablish the equilibrium after possible hydraulic and load shocks, constituting a robust system for the treatment of cassava starch wastewater. In these conditions the COD and solids removal remained satisfactory. After a 450-day reactor running, no changes were observed in the composition and structure of the support material, an indication that bamboo and PVC are possible cheap and efficient alternatives for biomass immobilization.
Highlights
Cassava starch is considered a product of great importance due to its functional properties which make it applicable to the most varied sectors of the economy
The statistical analysis was based on the split plot design in which the OLR corresponded to the main plot and the support material to the subplot
13 assays (A1 to A13) were carried out in which the increase in the organic loading rate was due to hydraulic retention time (HRT) reduction and variation of organic matter (COD) in cassava starch wastewater
Summary
Cassava starch is considered a product of great importance due to its functional properties which make it applicable to the most varied sectors of the economy (food, pharmaceutical, chemical, textile, paper among others). In the transformation process of cassava into starch there is wastewater generation, resulting from operations of starch extraction and root washing Anaerobic treatment of cassava starch wastewater has been studied for biogas production (methane or hydrogen + methane) (Wang et al, 2011; Rosa et al, 2014; Kuczman et al, 2017), great part of cassava starch industries uses the lagoon system, where the produced gases (CO2, CH4, and H2S) are emitted to the atmosphere. The CLD allows the biogas valuation, this alternative presents some disadvantages as low methane productivity and continual cover maintenance (Cantrell et al, 2008)
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