**4. Anaerobic treatment of cannery wastewaters**

This class of biological wastewater treatment technology utilizes microorganisms to degrade the organic pollutants in the absence of oxygen. The sludge in the anaerobic biological reactor consists of anaerobic bacteria and other microorganisms. Food processing wastewaters are particularly suitable for anaerobic treatment processes, firstly because of their high organic load and secondly because they rarely contain toxicants or inhibitory compounds [2].

### **4.1 Up-flow anaerobic sludge blanket (UASB)**

The UASB process makes use of suspended growth biomass, but the gas-liquidsolids separation system is integrated with the bioreactor. The operating conditions within the reactor could be adjusted to allow the formation of large, dense, and readily settleable particles that can lead to the accumulation of very high concentrations of SS, on the order of 20 to 30 g/l as VSS. These high suspended solids concentrations allow significant separation between the SRT and HRT, and operation at relatively short HRTs, often on the order of two days or less, even when the SRT is long. The three-phase UASB reactor allows the achievement of compact and cheaper units due to its ability to separate the gas, water, and sludge mixtures under high turbulence conditions. It has multiple gas hoods for the separation of biogas and can be operated in a one-metre-height reactor that prevents the formation of floating layers [9]. Due to the extremely large gas/water interfaces, the turbulence is greatly reduced, so it is possible to operate the treatment process with relatively high loading rates of 10–15 kg/m3 d [8].

UASB technology is known for its efficiency in treating wastewaters with high carbohydrate content. In this respect, the wastewater effluents from the canning industry are efficiently treated by microbes to produce a nutrient-rich starting material for anaerobic hydrogen production. This has led to the wide application of up-flow anaerobic sludge blanket (UASB) reactor for the treatment of the wastewater effluents from food processing plants [10]. These reactors are well known for their ability to withstand variations in wastewater quality and complete shutdown of the reactor in off season [11].

Anaerobic treatment of a highly alkaline fruit-cannery lye-peeling wastewater was investigated, using an up-flow anaerobic sludge blanket (UArSB) reactor. Only a short initializing period was required before COD reduction and OLR had stabilized at 85 to 90% and 2.40 kg COD/m3 d, respectively. With subsequent increases in OLR to 8.1kgCOD/m3 d, the COD reduction remained between 85 and 93% and

biogas production peaked at 4.1 l/d (63% methane). After 111 days, the COD and reactor pH started to decrease and the gas production was reported to decrease after 102 days and continue to decrease to reach the lowest value of 0.93 l/d after 129 days. Subsequent reductions in the OLR, by reducing influent COD, had no effect on reactor stability. This reduction in the reactor performance was attributed to the inhibition of methanogenesis due to the sodium accumulation of sodium (potentially >20,000 mg/l) in the biomass [10, 12].
