Environmental Water Treatment Knowledge: The Meaning and Role of Domestic Sewage (Wastewater Discharged in Residents' Daily Lives)
Sewage generated in the course of human life is one of the principal sources of water body pollution, consisting mainly of fecal and washing wastewater. The daily volume of domestic sewage discharged per urban resident ranges from 150 to 400 L, closely related to the standard of living. Domestic sewage contains large amounts of organic matter such as cellulose, starch, sugars, and fat-protein; it also frequently contains pathogenic bacteria, viruses, and parasite eggs, as well as inorganic salts including chlorides, sulfates, phosphates, bicarbonates, and sodium, potassium, calcium, magnesium, and the like. Its general characteristics are high nitrogen, sulfur, and phosphorus content, and under anaerobic bacterial action it readily produces foul-smelling substances.
It mainly originates from urban domestic sewage, hospital wastewater, garbage, and surface runoff. The characteristics of pathogenic microorganisms are: 1) large quantities; 2) wide distribution; 3) long survival time; 4) rapid reproduction; 5) easy development of resistance, making them hard to eliminate; 6) after conventional secondary biochemical wastewater treatment and chlorination disinfection, certain pathogenic microorganisms and viruses can still survive in large numbers. Such pollutants actually enter the human body through multiple pathways, survive within it, and cause human disease.
The common feature of these organic substances is that, once they enter a water body directly, they are decomposed by the biochemical action of microorganisms into simple inorganic substances - carbon dioxide and water - consuming dissolved oxygen in the water during decomposition. Under anaerobic conditions, the pollutants undergo putrefactive decomposition and deteriorate water quality; such organics are commonly called aerobic (oxygen-demanding) organics. The more aerobic organics a water body contains, the more oxygen is consumed and the worse the water quality, indicating more severe pollution.
Building on this, industry experts have also carried out extensive research and improvement.
Along with the rapid development of the wastewater treatment market, China's wastewater treatment output also ended its long period of stagnation and achieved high-speed growth. China's wastewater treatment output grew from 460,000 tons in 2000 to 2.36 million tons in 2004, with an average annual growth rate of 82.6%; its share of domestic market demand rose from 24.47% in 2000 to 52.80% in 2004. Over the same period, global wastewater treatment output grew at only about 6%.
Starting from the late 1990s, China's state-owned and joint-venture enterprises such as TISCO, Baosteel, Baoxin, and Zhangpu built a series of wastewater treatment production lines through technology import and transformation, bringing wastewater treatment process technology and equipment to internationally advanced levels and giving initial scale to the industry. The product mix also changed positively and product quality improved rapidly. In particular, domestic cold-rolled wastewater treatment sheet grew rapidly: in 2003 domestic cold-rolled sheet output reached 1.7 million tons, exceeding imports for the first time, with a self-sufficiency rate of 66%; in 2004 domestic cold-rolled sheet output reached 2 million tons with a self-sufficiency rate above 70%. From end-2004 to end-2005, domestic cold-rolled wastewater treatment capacity will increase by about 1.5 million tons, basically meeting domestic demand. By 2007, China will become a net exporter of wastewater treatment products.
(Challenge 1) Population Growth Brings More Sewage
In China, as urban population grows and industrial and agricultural production develops, sewage discharge keeps increasing and water pollution is fairly severe, spread almost nationwide. By the end of 2000, among 663 cities with established administrative status, 310 had built wastewater treatment facilities, with 427 wastewater treatment plants constructed, an annual treatment volume of 11.36 billion m3, and a treatment rate of only 34.23%.
Under the socialist market economy, wastewater treatment begins with a certain amount of capital investment. The scale of wastewater treatment capital determines the scale of treatment. The growth rate of the capital itself determines the speed of wastewater treatment development and the pace of technological progress. In real-world treatment, decision schemes with advanced technology and low treatment cost usually require larger upfront capital. In this sense, the faster the capital itself grows, the faster the progress and application of wastewater treatment technology, and the faster the treatment develops.
In addition, practical engineering applications require consideration of multiple factors.
Under the socialist market economy, part of urban wastewater treatment funding must also receive necessary subsidies from the government, for various reasons. Mainly: 1) wastewater treatment generally has low price-demand elasticity and government 'monopoly' operation, so fee setting must consider residents' affordability and cannot rely solely on competitive pricing to fully resolve facility construction and enterprise development; 2) the service provided by wastewater treatment is public in nature, and the use of many facilities is hard to meter, so service fees cannot directly enter the market as equivalent exchange but only become part of public consumption; 3) the service has broad sociality and external economies, so when measuring investment returns, social benefit comes first.
Furthermore, this technology has also seen wide application and practice in related fields.
Under China's current practice, shares are classified by investor identity into state shares, legal-person shares, individual shares, and foreign-capital shares; there is no division between preferred and common shares. China's Company Law contains no concept of preferred shares and makes no corresponding provision. This is because China's joint-stock enterprises were transformed from enterprises under the planned economy, carrying various historical traces that remain legacy issues to be further explored and resolved in reform. Starting from the reality of urban wastewater treatment, we can explore issuing wastewater treatment stocks. This requires joint-stock restructuring of existing wastewater treatment enterprises, issuing some preferred shares to domestic and foreign private capital, or transferring some state shares to private capital in the form of preferred shares; the funds raised are used by the wastewater treatment enterprise for treatment. Based on existing enterprises' performance and with continued post-restructuring growth, this approach has a higher chance of successful fundraising.
Used to remove impurities such as solid suspended matter, grease, and colloids from industrial and urban sewage, it can, with the help of chemical flocculants, minimize by-products generated in the sewage and allow recovery and reuse. In the paper industry, the machine treats paper-machine white water and intermediate wastewater such as de-inking effluent, recovering fiber on one hand and enabling reuse of treated water on the other, greatly reducing environmental pressure. Designed after a popular U.S. prototype from the 1990s, it features advanced technology and simple structure and is currently the most advanced domestic wastewater treatment equipment.
As living standards rise, domestic sewage discharge becomes increasingly severe. Under such circumstances, domestic sewage treatment processes keep improving; below we look at the latest treatment process flows.
Wastewater Treatment Process Brief: The biological aerated filter places packing in a biological filter unit and supplies artificial oxygen so that large numbers of microorganisms grow on the packing. This process equipment consists of a filter bed, air distribution device, water distribution device, and drainage device. The aeration device uses a dedicated aeration head; the medium-to-small bubbles produced are repeatedly cut by the packing, approaching the effect of micro-aeration. Because of the high sludge concentration in the reactor and compact facilities, it greatly saves footprint and reduces reaction time.
Wastewater Treatment Process Brief: The main cause of water eutrophication is that humans discharge large amounts of ammonia nitrogen and phosphorus into water bodies, with phosphorus being the most important factor. Throughout domestic treatment processes, phosphorus removal has long troubled plant operation. Traditional physico-chemical phosphorus removal requires large amounts of chemicals and has high operating costs and large sludge yields; pre-anoxic biological phosphorus removal has low operating cost but, relying entirely on microbial phosphorus uptake and release, struggles to meet national process requirements, and is even harder to satisfy when reclaimed-water reuse is considered.
Wastewater Treatment Process Brief: Because China's small towns have dispersed settlements and many low-volume pollution sources, town-level plants are mostly below 10,000 t/d. Processes commonly used in large and medium cities - conventional activated sludge, A2/O, SBR, oxidation ditch - would, if used for small towns, incur persistently high operating costs and fail to run normally. Processes must suit small towns with low investment, low operating cost, stable and reliable technology, and relatively simple operation and management.
The rotating contact oxidation process is a new generation of aerobic biofilm treatment technology developed from the rotating biological contactor, combining the advantages of biological contact oxidation. The rotating contact oxidation process and complete equipment provide a simple and reliable treatment method. In the whole system, the rotating shaft is the only moving part; once the machine fails, ordinary mechanics can repair it. The system biomass auto-compensates with changes in organic load. The microorganisms attached to the discs are living: when organic matter in the sewage increases, they increase, and vice versa. Thus the system's performance is not easily affected by sudden flow or load changes and power outages. Operating cost is low - only one-eighth to one-third of other aerated systems' power use. Footprint is only about half that of conventional activated sludge. Because of the diverse microorganisms in the biofilm, it efficiently treats various refractory industrial wastewaters.
China's economic levels vary greatly by region; cities with lagging economies cannot devote much funding to pollution control, so how to use limited funds to reduce environmental pollution is a problem for many city governments. In treatment, until recently some cities used primary or enhanced-primary processes whose effluent failed the national secondary discharge standard for organic pollutant removal. The cyclic intermittent aeration process fully leverages the high efficiency of high-load oxidation ditches and the good effluent of sequencing batch activated sludge, ensuring the system meets the national primary discharge standard for organic pollutant removal. Its investment and operating cost are about 30% lower than typical secondary biological systems focused on organic removal, suiting China's current-stage treatment needs.
During treatment, China's major rivers and lakes suffer severe eutrophication from phosphorus pollution; the NEPA set relatively strict phosphorus discharge standards to control it. Enhanced biological phosphorus removal integrates chemical and biological phosphorus removal, mainly removing organic pollutants and phosphorus in all forms. Through anaerobic digestion, the activated sludge in the biological system produces volatile organic acids as substrate/nutrient for phosphorus-accumulating organisms (PAOs), enabling their selective proliferation and return to the biological system so it operates in a high-efficiency phosphorus-removal state; meanwhile, the phosphorus released by sludge under anaerobic conditions is eliminated by chemical removal. This is an efficient municipal treatment technology meeting China's current need - to further remove phosphorus beyond conventional secondary treatment to solve eutrophication.
The CCAS process - Continuous Cycle Aeration System - is a continuous-feed SBR aeration system, improved from SBR (Sequencing Batch Reactor). The SBR process was developed as early as 1914 but was hard to apply in large plants due to cumbersome manual operation, backward monitoring, and easily clogged diffusers; it was long considered suitable only for small plants. After the 1960s, automatic control and monitoring advanced rapidly and non-clogging micro-pore diffusers were developed, enabling widespread intermittent treatment. In 1968, Australia's University of New South Wales and U.S. ABJ Inc. jointly developed the 'continuous inflow, periodic discharge, extended-aeration aerobic activated sludge process using an intermittent reactor system.' In 1986 the U.S. EPA formally recognized CCAS as an Innovative/Alternative (I/A) technology, becoming the most advanced computer-controlled biological phosphorus- and nitrogen-removal process [1].
CCAS requires little pretreatment - only a 15 mm gap mechanical bar screen and grit chamber. The biological core is the CCAS reactor, where phosphorus removal, nitrogen removal, and degradation of organics and suspended solids are completed, with effluent meeting discharge standards.
Pretreated sewage continuously enters the pre-reaction zone at the front of the reactor, where most soluble BOD is adsorbed by activated-sludge microorganisms and then passes with them through openings at the lower partition wall into the reaction zone at low velocity (0.03-0.05 m/min). In the main reaction zone, the 'Aeration, Idle, Settle, Decant' program runs cyclically, so the sewage completes carbon removal and denitrification through repeated 'aerobic-anoxic' phases and phosphorus removal through repeated 'aerobic-anaerobic' phases. The duration of each step and the corresponding equipment operation follow a pre-set, adjustable program under centralized computer control.
The SPR wastewater treatment system first uses chemical methods to precipitate dissolved pollutants from true solution into colloidal particles or tiny suspended granules with solid interfaces; selects efficient, economical adsorbents to separate organic pollutants and color from the sewage; then uses micro-physical adsorption to coagulate various colloids and suspended particles into large dense flocs; and relies on hydrodynamic principles of swirl and filtration in its self-designed SPR high-turbidity purifier to rapidly separate flocs from water. The clear water passes through a self-formed dense suspended-sludge layer inside the tank, reaching tertiary treatment level and enabling reuse; sludge is highly concentrated in the thickening chamber and periodically discharged under pressure. With low moisture content and good dewaterability, the sludge cake can go directly to mechanical dewatering and, after dewatering, be used to make sidewalk tiles, avoiding secondary pollution.
Meanwhile, related supporting processes and equipment are also being continuously optimized and upgraded.
Although, thanks to rising attention to environmental protection from the state and governments at all levels, China's wastewater treatment industry is growing rapidly with total treatment rising yearly and urban treatment rates improving, by 2013 it was still in an early development stage.
On one hand, by 2013 China's treatment capacity still lagged the rapid expansion of water use, with pipeline and sludge-treatment facilities severely behind. On the other hand, China's treatment rate still showed a clear gap versus developed countries, and facility load rates were low.
Therefore China should improve wastewater treatment policies and regulations, establish a regulatory system, create a reasonable fee system, support domestic environmental industry development, and advance industrialization and marketization of the sector. Wastewater treatment is a sunrise industry with broad prospects. During the 11th Five-Year Plan, China will invest 300 billion yuan to advance urban treatment and reuse, ushering the industry into a high-speed development period.
After a price hearing on adjusting treatment fees and with municipal government approval, Guangzhou finalized its urban wastewater treatment fee reform. Residential sewage adopts tiered metering fees linked to Guangzhou Water's residential tiered metering. Specifically: Tier 1 base is household monthly use up to 22 t (inclusive), charged at 0.9 yuan/t; Tier 2 base is the 22-30 t (inclusive) portion, charged at 1.2 yuan/t; Tier 3 base is the portion above 30 t, charged at 1.5 yuan/t.
