The water consumption of an average household in industrialized countries is around 120-200 liters per person per day. The amount of wastewater that reaches a treatment plant can be as much as twice this figure due to stormwater and ground water that leak into the system. Many operators are struggling under these capacity pressures.
“When discussing investments in wastewater treatment, people often focus on operating costs. This, however, does not give a comprehensive view on cost,” says Bengt Hansen, Senior Specialist in Application Development at Kemira. “The initial capital investment in treatment technology, land and infrastructure also plays a significant part that needs to be considered.”
Historically, municipalities have calculated very long write-off times for investments, sometimes as long as 50 years. This refers to the expected calculated lifetime of the process. With these kinds of write-off times, the capital expenditure may appear rather low. In reality, no wastewater treatment plant is used in the same way for 50 years. It is likely that the plant requires renovations much sooner than that. This means that investment proposals have been approved on misleading terms.
“Compact plants tend to be most economical. Several studies and real-life examples have shown that chemical pre-treatment brings significant efficiencies both with regard to regulatory compliance and with cutting costs,” Bengt says. “As chemical pre-treatment reduces the organic load on the downstream biological process, you can expand the capacity of your treatment plant at a very low cost.”
Chemicals of course are a cost for the treatment plants, but when considering if you should build a large (less chemicals) or compact (chemical pre-treatment) site, it is most probable that a compact plant is more cost-efficient in the long-term due to lower investments. The combination of chemical pre-treatment and biological treatment utilizes the best of two methods.
Chemical treatment is the most efficient method to remove phosphorus from water. Efficient phosphorus removal with pre-precipitation is attractive also with regard to energy consumption – impacting further the operating costs and the CO2 footprint of the plant. Often, the cost of precipitation chemicals can be covered by the improved energy balance alone.
Pre-precipitation increases the amount of digester gas produced during anaerobic sludge stabilization. During conventional biological treatment, a large proportion of the particulate organic material is converted to carbon dioxide. During pre-pre¬cipitation, the particulate organic material remains intact in the sludge, which means that it can be converted to methane gas during the digestion process. This means that considerably more methane gas/biogas is generated when pre-precipitated sludge is digested than is the case with biological sludge. In best cases, treatment plants can turn energy-neutral or even energy-positive, eliminating one of the biggest operating expenses altogether.
“In best cases, treatment plants can turn energy-neutral or even energy-positive, eliminating one of the biggest operating expenses altogether.”
“Pre-precipitation has been introduced at many treatment plants to reduce the load on the biological stage. No conversion work is usually required, so costly investments can be avoided. At the same time, the capacity of the plant can increase significantly. The choice of process clearly has a big influence on the economics of wastewater treatment. The investment cost per volume, energy consumption, as well as chemical and sludge processing costs all impact the total lifecycle costs and should be taken into consideration when evaluating the different technologies. Most importantly, with chemical pre-treatment we can reduce the carbon footprint and improve the overall treatment results,” Bengt summarizes.