Many studies have examined the use of anaerobic technology on a lab-scale or pilot-scale level for the treatment of agro-industry wastewater. The studies encompass various types of wastewater such as starch, dairy, sugar, coffee, brewery, fish, slaughterhouse, and others. Several anaerobic treatment systems have been developed, including UASB, Anaerobic Hybrid Reactor, Anaerobic Digestion, Fixed Film Reactor, Anaerobic Fluidized Bed Reactor, and Anaerobic Baffled Reactor. These studies show that anaerobic technology is the most suitable and attractive option for treating high-strength organic wastewater. It has several benefits such as low energy consumption, less sludge production, and high OLR (organic loading rate). Additionally, energy can be recovered from biogas of the anaerobic process, making it a promising option compared to conventional aerobic treatment methods. However, the reactor design may require modification to improve the quality of the treated wastewater.
Many detailed reports have shown that anaerobic treatment is effective for treating agro-industry wastewater at full scale. It reduces organic pollution and lowers treatment costs. The first full-scale application of this technology was in the sugar industry in the mid-1970s, and it has since become a standard method for a wide range of industries. By 2001, over 2,000 anaerobic treatment plants had been installed in 65 countries, with the UASB process being the most common. However, anaerobic lagoons are not an ideal solution for environmental protection due to their large land usage, bad odor, high methane and CO2 emissions, poor removal efficiency, and solid waste accumulation. Anaerobic treatment is particularly successful in agro-industry, accounting for more than 87% of its application.
To improve the quality of treated wastewater, studies suggest combining anaerobic and aerobic treatment methods. This involves using stabilization ponds or constructed wetland systems as post-treatment to meet acceptable levels of COD, BOD, and nutrients for discharge.
In order to achieve high loading rates for industrial wastewater treatment, according to Frankin (2001), it is important to have short hydraulic retention times while still maintaining positive biomass retention. As a result, different reactor designs have been created to achieve these conditions.
Upflow Anaerobic Sludge Blanket (UASB)
The UASB reactor was developed in the Netherlands in the 1970s. It was one of the first systems in which a granular biomass was observed. It has excellent settling characteristics and a device that separates the biomass, biogas, and water. This system ensures excellent sludge retention. The main disadvantage is the long start-up period, particularly when proper seed sludge is not available in sufficient quantities. The first start-up phase would benefit from skilled operation.
Expanded Granular Sludge Bed (EGSB) and Internal Circulation (IC) Systems
A newer version of AnWT technology is the EGSB process, which is an upgrade from the UASB process. The EGSB system uses granular sludge and has been developed to address the issues encountered in a full-scale Fluidized Bed (FB) system that used carrier materials for biofilm attachment. The EGSB system relies on expanding gas and hydraulic liquid flow to operate, and the IC system is a more complex version of the EGSB concept. The EGSB and IC systems can handle high organic and hydraulic loading rates, and they are effective in treating low-strength wastewaters, even at ambient temperatures as low as 10°C.
Fixed Film or Anaerobic Filter
Another way of retaining biomass is by immobilizing it on a fixed carrier, which can be done in a fixed film system. This system can withstand toxic shock loads, but the drawbacks are its low loading potential and the high cost of the carrier material. (van den Berg et al., 1985; Frankin, 2001)
Fluidized Bed (FB)
The FB system was developed in the 1980s to immobilize biomass by forming biofilms on fluidized carrier materials like sand, activated carbon, and basalt. However, the system faced issues of excessive growth on the carrier under mild shear conditions at the top of the reactor and no growth under high shear conditions required to fluidize the carrier at the bottom of the reactor (Frankin, 2001).
The hybrid system combines features of the fixed bed and UASB systems, using granular biomass and fixed carriers. These systems can effectively treat high COD-loads and have potential for low strength wastewater.