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Wastewater Treatment: Beyond Pollutants removal
Zhang Guangming
Our editor from State Key Lab of Urban Water Resource and Environment, Harbin Institute of Technology, 73 Huanghe Road, Harbin, 150090, CHINA
Water is the origin of life, the mother of nature. Ancient cities in China, India, Egypt, Greece and other lands all originated from big rivers. Peoples built their houses along rivers, intake water from rivers and discharge the used water into rivers directly. That was the “use-discharge” age.
With the increase of population and development of society, wastewater treatment became necessary in most places. The used water was treated before discharge, pollutants removal is the main objective in this situation. Various wastewater treatment techniques have developed, including physical, chemical, bio-treatment methods. Activated sludge process and its modified types such as SBR and oxidation ditch are the most important ones. Such a “use-treat-discharge” model is still prevailing in the world.
Further social development made the simple model insufficient. More than 1.5 billion people lack sufficient water resource and the number is increasing quickly. Wastewater in more and more places becomes unconventional water resource, i.e. wastewater treatment must generate some clean water besides pollutant removal. The new model of “use-treat-reuse-discharge” has been developed with some variations. An extreme example is supplied by Singapore, a city country with virtually no water resource. Wastewater gets extensive treatments and then is used as drinking water resource; a major government project. Ne water is developed and the goal is to substitute 30% clean water resource. The north China has practiced in a small scale recharge groundwater with treated wastewater; similar pilot-scale tests were also tried in Berlin in 1990s. The commonest practice of the “use-treat-reuse-discharge” model is to use the treated wastewater for some purposes with low water quality requirements, such as irrigation in agriculture, cooling water in electric power plants, road and toilet flushing, car washing and so on. Now-a-days reclaiming water out of wastewater is a widely accepted concept and practice around the world.
However, people always want to kill two birds with one stone. Shortage of energy drives the development of energy recovery from wastewater. Anaerobic biodegradation for industrial wastewater with high organic load has long been practiced with a welcomed byproduct of biogas, a renewable fuel that attracts lots of interests and venture investments. Many techniques have been developed for this purpose and Up-flow Anaerobic Sludge Bed (UASB) and its variations are the dominating ones. Another way for getting biogas is anaerobic digestion of sludge, an annoying byproduct of wastewater treatment process. New techniques like thermophilic digestion, multiple-phase digestion and effective pretreatments promise to provide 30-100% energy for wastewater treatment plants from the sludge.
In 1980s, microorganism fuel cell (MFC) was proposed in USA and quickly got attentions of researchers around the world. The unique concept of MFC is to utilize microorganisms to generate electricity directly from wastewater while degrading pollutants at the same time. Papers on MFC exceed 1000 each year, but the road to industrial practice is still very long since the largest MFC system in the world is roughly 1.0 L. Another approach for energy recovery from wastewater rooted from the disciple of heating and ventilation. Heater engineers use heat pump to draw energy from the temperature difference between wastewater and the environment. In early 1980s, large heat pump stations which used industrial wastewater as low-temperature-heat-source were established in Nordic countries. Heat pumps provide 40% heating for buildings in Stockholm, in which 10% energy is from wastewater. Efforts have been devoted to anti-corruption pumps.
Beyond clean water and energy, the ungratified researchers want other useful materials out of wastewater. Compost from wastewater sludge composting has been commercial products in North America, Europe and some Asian countries. Hot topics include co-composting of garbage and sludge, engineering bacteria and novel compost tanks. Construction materials can also be made from sludge including cement, brick and shielding sheets. Sludge bricks were used for Japanese sidewalks in 1950s and have been encouraged in China since 1970s.
Since 1990s, synthesis of biodegradable plastic poly-hydroxybutyrate (PHB) by activated sludge in wastewater treatment systems has been regarded a promising alternative to cut the high cost of PHB synthesis from petro-products. Other valuable materials can be recycled from specific industrial wastewater including phosphorus and metals: Ni and Cu from electroplating effluent, Cr from leather wastewater, Mg, Mo, Co and Zn from effluents of mining, smelting, paint and other industrials. Such practices are widely welcomed due to economic benefits but sometimes cautions must be taken to protect the environment from secondary pollution in the resource recovery processes. A very promising approach with great potentials is to reclaim single cell protein (SCP) from nontoxic wastewaters. The United Nations estimated that about 500 million people suffer from protein shortage and most of them are kids in developing countries.
SCP can be utilized as human food supplements, but more importantly, it can be used in animal feed which can be efficiently transformed into high-quality proteins such as meat, egg, milk and fish. Worldwide demands for such high-quality food increase with the increasing population which brings tremendous pressure to water resource and soil. Use of certain microorganisms to transform organics, nitrogen and phosphorous in nontoxic wastewaters into SCP is a cost-effective method to meet the nutrients need without consumption of land, water, fertilizer, or pesticide. Photosynthetic bacteria (PSB), a group of common microorganisms in nature with unique dual-metabolism pathways, may be the answer. Most strains of PSB are nonpoisonous and contain 50+% SCP, coenzyme Q10, biopolymers, vitamin B12, antimicrobial agents, carotene, pantothenic acid and therapeutic compounds so they can be used as supplement in cultivation, medicine, cosmetic and food industry.
In 1950s, scholars in Japan successfully applied PSB to treat high concentration wastewater and recovered the biomass as fish feed. Further studies extended to different types of wastewater and sewage. High removal (more than 90%) of COD, N, P and S and high productivity of SCP (0.14 - 0.3 g-SCP/g-COD) can be achieved simultaneously. Let us talk about a single country, China. With 1.3 billion of people, China discharges 56.0 billion m3 of wastewater each year, in which COD is 13.5 billion kg. If 10% of the discharged wastewater can be treated by PSB with the lowest transformation efficiency of 0.14, we can obtain 0.19 billion kg of SCP.
Wastewater treatment has developed with the development of science and technology and the continual improvement of human needs. In the past fifty years, it has evolved from the basic pollutant removal to efficient energy and materials generation. Scientists and engineers from disciples outside environmental engineering join this area and new materials, new methods and new equipment are emerging endlessly. The future will be more exciting and promising when we think of wastewater treatment as an organic component of water, energy and material cycles.
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