Wastewater treatment is crucial to water management. But what happens to its byproduct, sludge? Experts in Wuhan City tested if sludge byproducts have commercial potentials.
Sludge, that oozing dark residue from wastewater, can be a resource, or so the scientists at Hubei University hypothesized. Acting on this theory, they embarked on experiments to find beneficial uses of sludge in 2006, and in the process found a strong contender for solving the city’s looming surplus sludge problem.
Sludge is a semi-liquid concentrate of pollutants and the end product of wastewater treatment. It usually contains human fecal matter and harmful microorganisms, and its proper disposal is a major concern for many modern cities, such as Wuhan in the People’s Republic of China (PRC).
The Hubei experiments, conducted in partnership with the Wuhan Urban Drainage Development Co., Ltd. (WUDDC)—the city’s public utility in charge of wastewater and drainage services—yielded two usable sludge byproducts
hydrolyzed protein liquid, which can be transformed into firefighting powder; and
hydrolyzed biosolids, which can be directly used in gardening and landscaping.
The next step was to scale up the lab experiment to determine how the sludge byproducts can be commercially viable, environmentally sustainable, and beneficial to Wuhan City. Scaling up, however, requires significant financing.
In 2008, WUDDC found a partner in the Asian Development Bank, whose facility for small water pilot and demonstration projects shared the scaling up cost. Completed in May 2009, the pilot study
tested the usefulness of the sludge byproducts the lab study generated
assessed through end-user survey and market study whether or not these byproducts are environmentally safe and financially viable.
With these studies’ successes, can Wuhan use its surplus sludge productively?
Halfway along the PRC’s great Yangtze River lies Wuhan City, the capital of Hubei Province and home to some 9.7 million people and many international corporations. Wuhan, the country’s sixth largest city, is fast becoming a financial, commercial, educational, and cultural center, as well as a transportation hub for railways and waterways. And like Shanghai, Beijing, Guangzhou, and other major cities, Wuhan is having its share of modern urban environmental problems, such as increased wastewater leading to heavily polluted rivers.
Since 2001, Wuhan has embarked on a series of wastewater projects through financial partnerships with ADB. A first phase loan, worth US$83 million, increased coverage of wastewater systems through the completion of sewers and wastewater treatment plants. A US$100 million second phase loan, covering wastewater and stormwater management, improved and expanded wastewater services in urban and suburban areas, reduced flooding in stormwater drainage areas, and increased institutional capacity of related implementing agencies.
To date, Wuhan city has 10 wastewater treatment plants capable of treating about 1.59 million tons of wastewater a day, with a total daily sludge production of approximately 500-600 tons. Currently, sludge is dumped at various suburban landfills, but this method can lead to leakage of toxins and other pollutants to the soil. These landfills will be shut down in a few years.
Surplus sludge is expected to be one of Wuhan’s biggest concerns in the coming years.
The disposal of sludge has always been one of the major environmental problems of cities and other urbanized areas. Before wastewater treatment plants, raw sludge from households and industries went directly to rivers, creeks and canals that acted as cities’ natural sewers. With more modern technologies, sludge is now treated and can be disposed of in a number of ways—dumping in landfills, direct soil injection, or even converted into something else through chemical processes.
In Sweden, for example, treated sludge is either used in land applications—as fertilizer, construction soil, and cover material—or for energy and biogas production. In Germany, treated sludge is incinerated with coal in thermal power stations, or with gas or oil in cement factories.
Throughout the PRC, sludge is disposed of in landfills together with other household solid wastes or used as fertilizer in agriculture, depending on the quality of the sludge and original effluents. However, because of substandard treatment of sludge and its increasing quantity, some landfill operators are now refusing sludge in their facilities, forcing wastewater treatment plant operators to store sludge in their plants, while others use illegal practices such as dumping sludge in remote areas or directly into the sea.
In Wuhan, the wastewater treatment plants produce sludge at a rate that can no longer be accommodated by the city’s landfills. Additional comprehensive sludge treatment facilities for wastewater treatment plants spell increased investments, which, together with operational costs, may be too steep, especially for smaller treatment plants.
Scientists at Hubei University used alkali hydrolysis, a chemical process involving the application of high temperatures, high pressure, and high concentrations of alkali, on sludge to extract protein liquid and reduce the sludge’s water content. The process also kills all unknown organic matters, including bacteria and viruses.
The bench-scale study revealed that sludge from Wuhan’s wastewater treatment plants is 75 to 85 percent water and contains high amounts of protein. This protein extracted from sludge can be further experimented on to identify various uses. What remains after hydrolysis is called “biosolids,” which can be used for gardening material.
For the pilot study, WUDDC transported 6 tons of sludge to a rented protein production facility. The 2-day protein extraction process produced 900 kilograms of concentrated liquid protein and about 3 tons of biosolids.
While the characteristics of the extracted protein were consistent with the results of the lab study, exploring its applications could not push through due to budgetary constraints. The project could not afford the bigger equipment and operation space needed for pilot-scale study.
Analysis from the Hubei Province Agricultural Technology Institute revealed that the biosolids’ heavy metal content is far below the agricultural limits, which means it can be used for gardening. The biosolids were then transported to a garden site where flowers were planted in 3 plots: one with natural soil, one with the biosolids, and one with a mixture of both. After 5 weeks of monitoring, all 10 plants were growing well in all 3 plots. No significant difference was noted between the plants growing in biosolids and those in natural soil.
To find out if there is a market for biosolids, a survey of end-users was conducted, mainly involving public and private gardening organizations. The survey found biosolids to be acceptable, with minor chemical adjustments required. It also revealed that end-users are willing to pay RMB80-180 per ton of solids. The financial viability study that followed the market survey also revealed that biosolids production, including transport costs, are estimated at RMB100-120 per ton, giving WUDDC roughly RMB20-30 of net profit per ton.
Apart from this, WUDDC would also save RMB50 per ton of sludge—the average fee required by landfill owners for sludge disposal. At the same time, absolutely no impact on the environmental is expected. Zero waste is achieved.
Meanwhile, further studies on the extracted liquid protein’s industrial applications and market value will be conducted as another project.
Presently, Wuhan is preparing for a third project, also to be financed by ADB, for solid waste and sludge management. Both the results and lessons from the Hubei University bench-scale experiments and the pilot-scale study will provide scientific and technical support to the project’s design.
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