3.6 Treatment of wastewater the sugar industry

Technical Description

The sugar industry is an important consumer of both drinking and industrial waters used in the refining process. Wastewaters produced have an high organic load and, initially in the refining process, also have an high particulate load. Thus, treatment of these wastewaters requires a process that combines mechanical, chemical, and biological treatment measures. The principle element of the purification process is based upon the aerobic activated sludge technology with one or more aeration stages.

The raw wastewater entering the treatment plant is mechanically treated (primary treatment) to remove coarse particulate, then conveyed to the primary separator which removes suspended particulate through sedimentation, and, finally, mixed with the recirculated, activated sludge in the aeration tank. The activated sludge is separated from the treated effluent in the secondary separator, and is recirculated to the aeration tank where it is again mixed with the wastewater entering the plant. Excess sludge is removed from the circuit at this time. Following this treatment, the treated effluent is discharged from the plant. This product water (the effluent from the treatment plant) is thus free of the major part of the degradable organic substances.

The principle factors which determine the ability of the purification process to remove organic pollutants, and, in that way, the quality of the effluent of the plant, are the reaction time between the wastewater and the biomass in the aeration tank, the type and speed of reactions that take place during wastewater treatment, and the concentration of the contaminants in the wastewater and the biomass in each moment during the reaction. Thus, for optimal treatment using aerobic activated sludge treatment, it is important that the inflowing wastewater have certain characteristics that make it treatable using this technology; namely, the wastewater must be biologically treatable (biodegradable), contain essential plant nutrients such as nitrogen and phosphorus (if these two elements are lacking, as in the wastewater from the sugar industry, they are added as urea and orthophosphate in the aeration tanks in order to ensure a C:N:P ratio of 100:5:1), be between 7.0 and 8.5 pH units (maintained using chemical reagents), and free of substances that inhibit the growth of aerobic microorganisms. In addition, the waste stream should be as constant as possible, in terms of both quantity and composition, in order to avoid the shocks that can negatively influence the purification process and the yield. Aerobic conditions must be maintained in the reactors (the lack of oxygen results in mineralization of the biomass), which also keeps the biomass in suspension.

In Romania, sugar processing wastewater is collected and transported through pipelines (preferable underground ones because the processing takes place during the cold season) to a collection facility, where it is passed through a sand separator to remove contaminants like sand, stalks, leaves, etc. From the sand separator, the wastewater passes by gravity flow to the primary separator, which is equipped with hydraulic devices for collecting the separated sludge and the foam. The separated sludge is removed, drained, and dried in sludge drying beds. The water passes from the separator into the first stage aeration tank, which is equipped with devices for aerating the effluent and adding the nutrients in order to promote the growth of active sludge bacteria which metabolize up to 75% of the organic pollutants in the wastewater. In the next phase, the water passes into the second stage separator from which the excess sludge is pumped to the drying areas, and the supernatant is pumped into an aeration tank, which further purifies the effluent by removing up to 95% of the initial organic load. The sizing of the aeration tanks should be such that they are adequate to handle the hydraulic charge and the appropriate concentration of the recirculated sludge, and provide sufficient aeration and sludge contact time.

From the aeration tank, the water passes in devices that collect the sludge, which is dried in sludge stabilization ponds, and remove the foam.

Extent of Use

This technology is used in Romania.

Operation and Maintenance

This is a locally available technology, which requires specialist operators. Nevertheless, the technology can be maintained at local industry level.

Level of Involvement

This technology is implemented at the local industry level.

Costs

The estimated cost of a facility capable of treating sugar industry wastewater at a rate of 300 to 350 m³/hour is approximately $12 million.

Effectiveness of the Technology

The technology removes 95% to 98% of the organic contaminants in sugar processing wastewater.

Suitability

The technology is suitable for all types of wastewater treatment where the wastewater has an high, predominantly organic, pollutant load which is biodegradable.

Advantages

This is an highly efficient means of wastewater treatment.

Disadvantages

No disadvantages have been identified.

Cultural Acceptability

This is an acceptable wastewater treatment technology.

Further Development of the Technology

When the technology is applied to the treatment other effluents, there is a need for a short study to determine the bioreaction conditions and contact times for each step.

Information Sources

Isvoranu Ioan, Food Chemistry Institute, Str Girlei 1, sector 1, Bucharest, Romania, Tel. (40-1) 679 20 40.