Analysis of conventional technologies for treatment of food processing wastewater

In advance of building a wastewater treatment facility, a food industry should undertake an in-plant waste control programme in order to minimise the use of water, for example for cleaning, transportation and cooling operations, etc. (Carawan et al., 1979). Within a specific food processing industry, there may be a range of approaches to achieve this; for example by substituting pneumatic transporting systems for water transporting and using nozzles that automatically shut off when released by the operator. Once appropriate in-plant control measures have been initiated, the food processor must then assess the projected strength and volume of the processing wastewater. These parameters are significantly influenced by the fluctuation in production volumes and foreseen production line expansion programmes. Hence, these factors must also be taken into account when designing the wastewater treatment system in order to ensure appropriate sizing and equipping of the system (i.e. adequate volumes, aeration systems, installed power, etc.). This again highlights the importance of properly characterising the wastewater stream to be treated. In addition, when considering the treatment of food processing wastewaters, their composition should be fully evaluated. Most wastewaters contain considerable amounts of suspended matter which can be removed by physical or chemical-physical processes. The soluble pollutants can then be removed by means of biological (both aerobic or anaerobic) treatments or chemical-physical processes (membranes or other). Figure 21.1 shows a possible, general, scheme for treatment of wastewaters from food processing. In this train of processes the following steps are considered:

1 Preliminary treatments. These generally consist of a screening step (maybe double) and eventual grit removal. The screens used could be vibrating, rotary or static. Usually the screens used have from 10 mm

Treatment Abattoir Wastewater

Fig. 21.1 Typical process scheme for treatment of wastewater from food processing.

Fig. 21.1 Typical process scheme for treatment of wastewater from food processing.

down to 1 mm openings. Material of small size can be removed by highspeed circular vibratory polishing screens. Screening systems may be used in combination to maximise the efficiency of the process. Efficiencies of these systems are variable: rotary drum and disc show removal percentages up to 40-50% for suspended solids.

2 Flow equalisation. Following the screening process and preceding the unit for suspended solids removal is a flow equalization step. Flow equalization is important in reducing hydraulic and organic loading in the biological process following. Equalization facilities consist of a holding tank and pumping equipment designed to reduce the fluctuations of the waste streams. The equalizing tank will store excessive hydraulic flow and stabilize the flow rate to a uniform discharge rate over a 24-hour day.

3 Primary sedimentation or flotation for suspended solids removal. After equalisation the elimination of suspended solids is carried out. This can be obtained through the application of a typical primary sedimentation process or by applying a dissolved air flotation (DAF) system. Primary sedimentation allows for the 30% and 60% removal of COD and total suspended solids, respectively (Metcalf & Eddy, 2002). DAF is probably the most common pretreatment for food industry wastewater: it can be used to remove oil, fats, greases and fine particles. The raw wastewater is brought in contact with a recycled, clarified effluent which has been pressurized through air injection in a pressure tank. The combined flow stream enters the clarification vessel and the release of pressure causes tiny air bubbles to form which begin their ascendancy to the surface of the water, carrying the suspended particles with them in their vertical rise.

At the top of the tank there is usually a mechanical apparatus to remove the floating skimmers. To improve the efficiency of solids removal, chemicals such as ferric chloride, alum, lime and anionic polymers, and acid adjustments to pH 5, are used. Varied combinations of alum and polymer, lime and ferric chloride, and acid adjustment, alum and polymer have been demonstrated to increase the particle removal efficiency of the DAF process. For example, when considering waste-waters from the meat industry, usually 40-50% of COD is due to coarse suspended matter (1 mm mesh), insoluble in water and slowly biodegradable; thus screening, settling and DAF are widely used to remove suspended solids and also fats, oil and grease. DAF units are usually assisted by adding chemicals, which permits the removal of a large amount of nitrogen, phosphorous and BOD (75-80%). Table 21.4 shows the typical performances obtained in DAF units for treating wastewa-ters from meat processing (Johns, 1995). As a drawback, this system may have considerable problems due to the long retention time and low surface-overflow rate - which leads to solids settling and the production of a large volume of putrefactive material, and difficulties in

580 Handbook of waste management and co-product recovery Table 21.4 DAF performances on slaughterhouse wastewater (from Johns, 1995)

Treatment efficiency

COD removal

TSS removal

Oil and grease removal (%)

Conventional DAF Acid, pH 4-4.5 DAF and chemical

40 71 31-92

60 78 70-97

90 93 89-98

addition dewatering. It is also important to note that this preliminary treatment can remove the carbon that may be necessary for the removal of nutrients in a later activated sludge process for biological nutrients removal (BNR; e.g. Bolzonella et al., 2001).

These preliminary unit operations (stage 1-3), operating all together, will generally remove up to 85% of the total suspended solids, and 5060% of the COD present in the wastewater.

4 pH adjustment. A pH adjustment is carried out after preliminary treatments in order to obtain a wastewater much more suitable for the biological processes following, generally in the neutral range.

5 Biological treatment processes (aerobic or anaerobic). In Europe most of the food processing plants deliver their wastewater to municipal systems after primary treatment, but in some cases the wastewaters may pass through a secondary biological (anaerobic or aerobic) treatment. In fact, to complete the treatment of the food processing waste-waters, the waste stream must be further processed by biological means. After adequate primary treatment, frequently used biological treatment systems include: anaerobic processes, extended aeration, aerated lagoons, trickling filters and land application. Recently, some new highly effective processes such as membrane bioreactors and jet loop reactors have come into operation. These processes will be presented and their performances discussed in Section 21.4.

6 Eventual tertiary treatment (membrane or other). After the biological step, treated water can be further polished by means of specific tertiary treatments such as the use of membranes (microfiltration, ultrafiltration and reverse osmosis; Cheryan, 1998), or other chemical-physical processes (activated carbon, precipitation, chelation) for the removal of specific pollutants and to further improve water characteristics.

When high standards for treated water are requested, the biological reactor is the core technology of the treatment process. The most common options for biological treatment are those already reviewed by Carawan et al. (1979) and reported in the following sections.

Treatment of food processing wastewater 581 21.3.1 Anaerobic treatments

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. Indeed, excluding equipment cleaning operations, for which chemicals and disinfectants are normally used, all the sources of wastewater are related to the preparation and processing of animal- and vegetable-derived raw material, and the wastewater characteristics are therefore mainly dependent on the nature of the organic matter processed. The upflow anaerobic sludge blanket (UASB) system has become the most widely applied reactor technology for high rate anaerobic treatment of industrial effluents (Fang and Chui, 1993; Lettinga, 1995; Driessen and Yspeert, 1999; Moletta, 2005). The main reason for the success of the UASB is its relatively high treatment capacity (up to 10 kg COD/m3 per day) compared with the other biological systems, which permits the employment of compact and economic wastewater treatment plants. In addition, anaerobic processes enable biogas production, with associated energy recovery, and are also characterised by low sludge production: typical yields are <0.1 kg volatile solids (VS)/kilogram of COD removed. However, these processes are not suitable for removal of nutrients and only a partial removal of nitrogen can be obtained in anaerobic conditions (Strous et al., 1997). Therefore, when high quality standards for treated water are requested, the anaerobic processes are generally coupled with aerobic processes such as the activated sludge process (see below). In UASB reactors effective sludge retention is achieved by use of a three-phase separator on the top of the reactor, which separates biogas, sludge and treated effluents. The biomass grows in the form of granules, which are easily settled. Although the sludge has good settling characteristics, sludge retention in UASB reactors becomes critical at high upflow gas and liquid velocities. The UASB process can cope with high-strength industrial effluents having a COD concentration in the range of 3000-10 000 mg/l or more (Lettinga, 1995). The organic loading and hydraulic capacity are the most critical design criteria for the upflow sludge bed reactor. While the reactor design for treating low-strength effluents is mostly hydraulically limited, for treating high-strength effluent the system is generally limited by its organic loading capacity. UASB reactors are usually operated at maximum upflow velocities of 1-3 m/hour or for minimal hydraulic retention times of 4-5 hours (Metcalf & Eddy, 2002). To overcome these limits a new type of reactor was designed, with an increased height/diameter ratio; these systems operate with higher upflow velocities and organic loading rates. An example of such a reactor is the expanded granular sludge bed (EGSB) which contains a granulated anaerobic active biomass (Lettinga, 1995; Nunez and Martinez, 1999). The internal circulation (IC) process consists of two UASB reactors, one on the top of another, working the first at high load and the second one at low loading (Demirel et al., 2005). Its

Table 21.5 Efficiency of UASB reactors for different food processing wastewaters

Wastewater

COD BOD5 Lipids Fat acids removal removal removal removal

Reference

Dairy effluent Winery

Slaughterhouse

Fruit juice

Vegetable processing Beer

Confectionery

Potato-chips

Sunflower oil factory Olive mill Clam processing

Coffee

Up to 97.7

93-99

Up to 80 80

57-80

92.4 82 87

80-85 56-77

70-78

91.5 86

67-82

70-94

81-95

Demirel et al., 2005 Brucculeri et al., 2005 Liu Victor et al., 2004 Austermann-Haun et al., 1997 Rintala and

Lepisto, 1997 Parawira et al.,

2005; Austermann-Haun and Seyfried,

1994 El-Gohary et al.,

1999 El-Gohary et al.,

1999 Saatci et al., 2003

Sabbah et al., 2004 Boardman et al.,

1995 Dinsdale et al.,

1997

special features are the separation of biogas in two stages and the internal circulation driven by the produced gas. UASB reactors are widely applied in the treatment of food processing wastewaters because of their capacity to remove BOD and COD at high levels and to recover methane, a renewable energy source that can be directly reused in the food processing plant. Table 21.5 shows the removal efficiency for BOD and COD from food processing wastewaters. As these are characterised by high levels of easily biodegradable soluble organic compounds, efficiencies are very high, generally >90%.

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