Anaerobic biological treatment of high strength cassava starch wastewater in a new type up-flow multistage anaerobic reactor

Abstract

Anaerobic treatment of cassava starch wastewater using an up-flow multistage anaerobic reactor was investigated. The results showed that startup was successfully accomplished in 22 d. The maximum 87.9% chemical oxygen demand (COD) was removed at hydraulic retention time (HRT) of 6.0 h at fixed concentration 4000 mg/L. In addition, 77.5–92.0% COD were removed as organic loading rates at 10.2–40.0 kg COD/(m³ d) at fixed HRT of 6.0 h. The Grau second-order kinetic model and modified Stover–Kincannon model were successfully used to develop a kinetic model of the experimental data. Furthermore, the specific methanogenic activity were 0.31 and $0.73\text{g}{\text{COD}}_{{\text{CH}}_4}/(\text{g}\text{VSS}\text{d})$ for the first and second feed, respectively. Finally, morphological examination of the sludge revealed Methanothrix spp. and Methanosarcina spp. were dominant microorganisms. All the results indicated that the UMAR could be used efficiently for treatment of wastewater containing high COD concentration from cassava starch processing.

Introduction

Biological treatment of organic polluted wastewater is an interesting process, and considerable effort has been made toward the development of more sophisticated and efficiency bioreactor in the area of wastewater treatment with activated sludge. Currently, aerobic and anaerobic wastewater treatment methods based on activated sludge are generally accepted. With appropriate analysis and environmental control, almost all wastewater containing biodegradable constituents with a BOD/COD ratio of 0.5 or greater can be treated easily by biological technologies. In addition, conventional aerobic technologies based on aerobic activated sludge processes are also dominantly applied for the treatment of domestic wastewater due to the high removal efficiency, the possibility for nutrient removal and the high operational flexibility (Gavrilescu and Macoveanu, 1999). However, there are at least two distinct disadvantages of aerobic process: their relatively high energy consumed and high excess sludge production, which requires handling, treatment and disposal (Leitão et al., 2006). Compared with the conventional aerobic technologies, anaerobic treatment of wastewater can serve viable and cost-effective alternative due to its relatively low construction and operational cost, no oxygen requirements, low nutrient requirements, low production of excess sludge, production of energy in form of biogas and so on (Kushwaha et al., 2011, Zhang et al., 2010). In particularly, anaerobic systems are suitable for the treatment of high strength wastewater (biodegradable COD concentrations over 4000 mg/L) (Chan et al., 2009). Therefore, highly polluted industrial wastewaters are preferably treated in an anaerobic reactor due to the high level of COD, potential for energy generation and low surplus sludge production. For example, Şentürk et al. (2010a) employed a thermophilic anaerobic contact reactor treating potato-chips wastewaters, and the COD removal efficiencies were found to be 86–97% as organic loading rates (OLRs) ranging from 0.6 to 8 kg COD/(m³ d). Zhang et al. (2010) reported full recycling the distillery wastewater produced for bioethanol production from cassava through a two-stage anaerobic treatment process. Tawfik and El-Kamah (2011) employed a combined system consisting of an anaerobic hybrid reactor, followed by sequencing batch reactor for treatment of fruit juice industry wastewater. Since anaerobic technologies have the advantage compare with aerobic technologies, thus, anaerobic technologies will play a major role in treating wastewater from all kinds of industry, in particularly, cassava starch industry.

Cassava, also known as tapioca, is a starch-containing root crop of worldwide importance as food, feed and non-food products. More than 70% of this production is produced by small-scale farmers in the subtropical and tropical regions between 30°N and 30°S of Africa, Latin America and Asia (Jansson et al., 2009). Cassava is a major source of food feeding for more than 700 million people in tropical developing countries and it is cultivated in a total global area of 18.6 million ha with a total production of 238 million tonnes (Patil and Fauquet, 2009). The status of cassava cultivation today is changing from subsistence farming to an industrialized system designed to process cassava into a diverse spectrum of products (Jansson et al., 2009). Particularly, cassava is mainly used to extract starch because starch is the principal reserve polysaccharide in plants. It was worthy noting that more than 600 million tons of cassava each year was harvested for extraction starch in Guangxi of China since 2009. Simultaneously, wastewaters were generated from about 150 existing cassava plants by the starch extraction process, which essentially involves preprocessing of roots, starch extraction, separation and drying. The process generates 20–60 m³/metric ton of wastewater with low pH, high chemical oxygen demand (COD), high biochemical oxygen demand (BOD) and suspended solids (SS) by nature (Annachhatre and Amatya, 2000, Annachhatre and Amornkaew, 2001). Although starch processing plants may produce diluted wastewater, it is a source of pollution and causes environmental problems, such as high toxicity to tropical duckweed (Bengtsson and Triet, 1994). Thus, effective technologies are necessary for the treatment of cassava starch wastewater.

A number of anaerobic biological systems, such as up-flow anaerobic sludge blanket (UASB) reactor for sewage and starch wastewater treatment (Mahmoud, 2008), internal circulation (IC) anaerobic reactor for swine wastewater treatment (Deng et al., 2006), horizontal flow filter with bamboo for cassava wastewater and anaerobic ponds (Rajbhandari and Annachhatre, 2004), have been used in the treatment of wastewater. However, in practical applications, all the anaerobic treatment suffers from the low growth rate of the microorganisms, a low settling rate, process instability, high hydraulic retention time (HRT) and the need for post treatment of the noxious anaerobic effluent which often contains ammonium ion and hydrogen sulfide (Chan et al., 2009, Ndon and Dague, 1997).

In the present work, a new type of up-flow multistage anaerobic reactor (UMAR) was designed to overcome the shortcomings of existing anaerobic reactor, and then it was applied to treat high strength cassava starch wastewater after removal of suspended solids. Effect of UMAR operating parameters such as flow rate, OLR and influent COD concentration were investigated in detail. The variation of pH values and methane production were also determined. Results were evaluated by using Grau second-order kinetic model and modified Stover–Kincannon model for multi-component substrate degradation. Moreover, the specific methanogenic activity (SMA) was also determined. Finally, the morphological and microbial structure of the granules was studied during the course of cassava starch wastewater treatment at stable state. This study will not only help to understand the process and performance of UMAR for treatment cassava starch wastewater, but also contribute to provide information for treatment of other high strength industrial wastewater with UMAR.