Fruit and vegetable processing plants generate large volumes of high strength wastewater that cannot be discharged without pretreatment.
Fruit and vegetable process plant wastewater typically contains discarded fruits and vegetables, soil particles, fruit and veg pulp and fibers, cleaning agents, blanching agents, salt and residues of pesticide. In addition to that, processing onions and ready made meals produces foul smelling wastewater. This unique and complex effluent requires specific and effective treatment. In this article we discuss some of the processes and technologies a wastewater treatment plant uses in order to produce cleaner fruit and vegetable process effluent.
Good practice in waste management reduces volumes of wastewater in a fruit and vegetable process plant by up to 95%. Such pollution control measures include dry cleaning and peeling of raw materials and recirculation of process wastewaters. This can significantly reduce the cost of wastewater treatment for a fruit and vegetable process plant and benefit the environment too.
Selecting the most suitable treatment for fruit and vegetable process wastewater depends on three main factors:
- the characteristics of the effluent
- the site conditions
- the economic viability of the proposed treatment plant
Therefore the configuration of a wastewater treatment plant will be specific to the unique requirements of the fruit and vegetable process plant.
It is important that the relevant analyses and surveys are carried out on the effluent in order to design the most suitable treatment plant. These tests will often include BOD, pH, temperature, suspended solids, volatile solids, settleable solids, and FOG. Often, the wastewater treatment specialists in charge of the job will design and implement a pilot scale plant to determine the best course of treatment for the fruit and vegetable process effluent. Adjustments can be made until the pilot scale plant is meeting the target effluent standards, at which point the full scale treatment plant is ready to commission and install.
Generally speaking, the most common treatment processes for food process effluent involve screening, pH adjustment and biological treatment. Production in a fruit and vegetable processing plant is often seasonal and it is for this reason that the wastewater treatment system needs to be robust and flexible. The basic pattern for treating wastewater of the seasonal kind is regulation, aeration, and settling.
Pretreatment of Fruit and Vegetable Process Effluent
Pretreatment is a term that covers a wide range of different processes in wastewater treatment. Some common forms of pretreatment of fruit and vegetable process effluent include flow equalization, screening, gravity separation of floatables and solids, dissolved air flotation (DAF), chemical treatment to supplement gravity separation or DAF, and biological treatment.
Flow equalization and neutralization reduce hydraulic loading and correct pH. The equipment includes buffer tanks, pumping equipment and air blowers which serve to mix the effluent, balance fluctuations in the waste stream and adjust the pH levels. Buffer tanks uniformly feed the flow to the treatment facilities or store the effluent for recycling/reuse elsewhere in the fruit and vegetable processing plant.
Screening removes larger solids and particles from the effluent by filtration through a mesh. In terms of fruit and vegetable process effluent, vibrating and rotary screens are most effective. Screening lowers suspended solids, settleable solids and reduces BOD discharge. It prevents clogging of municipal sewers and prevents damage to onsite treatment equipment. Screening also makes the downstream treatment processes more efficient. Currently, the most popular screens for fruit and vegetable process wastewater are vibrating/oscillating screens and rotary drum screens. Screening is the most inexpensive form of pretreatment.
Another common method of pretreatment of fruit and vegetable process effluent is separation of the floatables and settleable solids. This is usually by gravity or by air flotation. Certain waste streams may require the addition of chemical additives such as Al2(SO4)3, FeSO4 and FeCl3 to enhance the separation during DAF. This can reduce the BOD concentrations in the wastewater significantly enough to finally discharge to a municipal plant.
Biological Treatment of Fruit and Vegetable Process Wastewater
The use of biological secondary treatment is often necessary in order to meet local wastewater effluent standards. For fruit and vegetable process effluent, there are several different systems that can provide secondary treatment and the choice is largely dependent on the characteristics of the waste stream. Often, a combination of techniques will provide the best result. Some of these secondary processes include anaerobic lagoons, aerobic lagoons, faculative ponds, aerated lagoons, trickling filters, activated sludge and rotating biological contactors. As already mentioned, the choice will depend entirely on the characteristics of the effluent, the site conditions and the economic viability of the proposed treatment plant and associated technologies.