Cost-effective wastewater treatment

THE COHRAL technology from Global Water Engineering is an economical means of increasing the quality of wastewater produced by agribusiness whilst addressing the odour pollution issue toward assisting crop and livestock operations to achieve higher environmental standards. It involves a Covered High Rate Anaerobic Lagoon (COHRAL) that uses concentrated anaerobic bacteria to digest at least 70% of the organic matter (COD or chemical oxygen demand) in wastewater to produce effluent of far higher quality than open lagoons.

With GWE anaerobic and complementary technologies contained in totally enclosed tanks, or reactors, the water quality can be further raised to exceed 90% COD removal or even up to 99% in some instances, says CEO Jean Pierre Ombregt. The company has been a world leader in clean water and green energy solutions for more than 35 years.

COHRAL, including the first example in Australia now being engineered by CST Waste Water Solutions, which represents GWE technologies in Australasia, eliminates many of the odours associated with open lagoons often used in meat, dairy and crop waste processing. Further, the methane gas produced in covered anaerobic lagoons and totally enclosed anaerobic reactor systems can be collected and used to feed steam boilers and processing plants, thereby replacing fossil fuel, says Michael Bambridge, managing director of CST Waste Water Solutions.

The first COHRAL installation in Australia, which will be employed at a meat processing and packing plant, will feature reuse of the biogas in the boilers. Closed lagoons not only prevent methane from escaping into the atmosphere where it is many times more damaging to the atmosphere than C02 emissions but also generate energy rather than being heavy consumers of energy in processing and oxygenation.

COHRAL covered lagoons

According to Mr Bambridge, open lagoon wastewater treatment systems were popular because they were simple, cheap and easy to operate. They are, however, ineffective in controlling algae and suspended solids in warm weather and removing pollution load.

COHRAL installations represent a major advancement over open lagoons and are suitable for solutions involving less technology-intensive applications that allow a long process residence time and where adequate space is available. They are a cost-efficient first step toward totally enclosed reactor type anaerobic technologies.

The anaerobic lagoons consist of two zones, with the complete surface of the lagoon covered with an influent distribution system. The first and largest zone receives the major part of the incoming wastewater. This reaction zone is where the anaerobic digestion occurs.

The second, smaller part serves as a post-digestion and pre-settling zone where a partial clarification of the effluent wastewater takes place. Settled sludge collected in this zone is pumped back to the inlet of the lagoon.

Part of the anaerobic effluent is recycled back to the lagoon. The remaining effluent of the lagoon flows by gravity towards complementary technology such as the GWE proprietary SuperSep-CFS separation technology used in the first Australian installation.

The COHRAL system leverages simplicity and energy efficiency, with no additional mixing facilities required in the anaerobic lagoon. The influent distribution system acts as a hydraulic mixing system, converting a standard low load/low efficiency lagoon into a COHRAL system with increased efficiency.

Safety is also integral to the system. Each anaerobic lagoon is covered by a special floating membrane to retain the methane produced. It operates at zero biogas pressure (-1 to +1 mbar range). A sensitive membrane level measurement system controls the speed of a biogas extraction fan bringing the gas at 20 mbar to go to the flare. Compared with “inflated” single membrane covers, the risk for leaks is virtually zero.

Simplicity extends to the Scada computer control system typically employed with the COHRAL technology. It handles all indications and alarms, daily plan operation reporting and “trending”, and all motor start/stop and auto/manual functions.

The future of uncovered lagoons

The character, toxicity and quantity of agribusiness contaminants today are far more complex than those in the past, with pathogens, nutrients and highly publicised problems such as blue-green algae spilling into the environment from lagoons and toxins building up over decades. Population growth is taxing reserves of shared community resources such as clean water, clean air and land for urban development and sustainable agricultural uses, says Bambridge.

Food and beverage processors will be in the front line of change, along with primary processors such as those for meat and dairy and operators of any industrial manufacturing or processing plant that draws water from its surroundings and returns water to the environment.

Solutions do not have to be expensive and can be done in stages. The first step can involve covering lagoons and incorporating anaerobic processes with properly engineered feed and recycle systems such as those in the COHRAL process. The second step can be the use of tanks to contain anaerobic and other processes and minimising land use, reducing plant footprints and providing high security against leaks and groundwater contamination. Anaerobic processes can also be more closely efficiently controlled in such close environments, optimising water purification and green energy production. A third stage can be the incorporation of the most advanced anaerobic technologies into sealed tank environments such as GWE’s RAPTOR treatment system for organic residues, which can convert almost any organic residue or energy crop into biogas, valuable electricity or heat.

Anaerobic digestion facilities have been recognised by the United Nations Development Programme as one of the most useful decentralised sources of energy supply as they are less capital-intensive than large power plants. They can also benefit local communities by providing local energy supplies and eliminate the need for large and often smelly and environmentally challenging settling lagoons.