Recovery, reuse to reshape wastewater treatment plant designs

The new basis of design and refit for all wastewater treatment plants must be the aim of not only making the plants more efficient, but also extracting and reusing the energy nutrients and potable water found in wastewater, says the University of California Department of Civil and Environmental Engineering’s Professor Emeritus George Tchobanoglous.

“Wastewater is a renewable source of energy and nutrients. In the twentieth century, we aimed to get rid of these. In the twenty- first century, we must recover the energy, nutrients and potable water,” he says.

Tchobanoglous advocates the use of existing wastewater treatment structures, but with design principles based on improving the energy efficiency of these plants and recovering energy, nutrients and water.

“We are still using the same [wastewater treatment plant] designs of 100 years ago. This has had a negative impact on many treatment plants, as the unintended consequences of old designs have impacted on current designs, and will have an impact on future designs. In the twenty-first century, we must design and refit plants to enable the sustainable reuse of water and wastewater. We cannot use water only once.”

The energy required to treat wastewater is between 1 200 MJ/m3 and 2 400 MJ/m3, while the thermal and chemical energy contained in wastewater is about 6 000 MJ/m3, he highlights.

“The nature of the future of the field [focuses on how] we can extract energy from the wastewater. This requires a rethink of how we treat wastewater, especially with regard to energy efficiency. Should we just accept the characteristics of wastewater flowing into the treatment plant? We can filter out some of the organic solids and place the solids in an anaerobic digester to generate energy, but the removal of nitrogen and phosphorus from the water may reduce the effectiveness of certain subsequent treatment processes,” says Tchobanoglous.

A screening modification of existing plants will reduce solids and will alter the particle-size dis- tribution, impacting on settlement rates, and can enable certain stages of treatment to occur faster. Changes to existing wastewater treatment plants can increase their capacity by twofold to fourfold, he notes.

“However, consideration must be given to return flows from these new stages in the treatment process. Removing phosphorus as struvite can help to mitigate the ammonium and phosphate in return flows produced by the anaerobic digesters,” he avers.

Tchobanoglous emphasises that the costs of treatment plants must be kept low and the plants must be efficient to achieve this.

“We are reducing the quantity of water flowing in the pipes, but this increases the concentration of waste and the production of hydrogen sulphide, which corrodes the pipes. Also, the low flow rate means that gravity-flow designs do not apply and solid waste accumulates in the pipes,” he explains.

“Water reclamation and reuse have become an attractive option for conserving and extending available water supply by potentially substituting reclaimed water for applications that do not require high-quality drinking water. This augments water sources and provides an alternative source of supply to assist in meeting present and future water needs and in protecting aquatic ecosystems by decreasing the diversion of fresh water.

“Water reclamation can also reduce the quantity of nutrients and other toxic contaminants entering waterways and the need for water control structures, such as dams and reservoirs, and is also compliant with environmental regulations [in that it manages] water consumption and wastewater discharges [better],” he concludes.

 

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