What a City Flushes Away: Load, Pathogens and the Process Choices That Follow

Domestic sewage is mostly what people produce in a day: faecal matter and wash water. Per urban resident that runs 150 to 400 L, and the number tracks living standard closely. Chemically it is cellulose, starch, sugars and fat-protein, plus pathogenic bacteria, viruses and parasite eggs, plus inorganic salts - chlorides, sulfates, phosphates, bicarbonates and sodium, potassium, calcium, magnesium. High nitrogen, sulphur and phosphorus is the signature, and under anaerobic bacterial action it turns foul quickly.

The Pathogen Problem Nobody Sees in the Permit

Pathogenic microorganisms arrive from urban domestic sewage, hospital wastewater, garbage and surface runoff. They are numerous, widely distributed, survive a long time, reproduce fast, and develop resistance easily, which makes them hard to eliminate. After conventional secondary biochemical treatment and chlorination disinfection, some pathogenic microorganisms and viruses still survive in large numbers, and they reach people by several pathways.

The organic fraction behaves differently. Discharged straight into a water body, organics are broken down by microbial biochemical action into simple inorganic substances - carbon dioxide and water - consuming dissolved oxygen on the way. Under anaerobic conditions the same pollutants putrefy and water quality deteriorates. These are the aerobic, oxygen-demanding organics: the more of them a water body carries, the more oxygen is consumed and the worse the water.

Growth, Capital and the Treatment Gap

As the wastewater treatment market expanded, China's treatment output came out of a long stagnation and grew fast - from 460,000 tons in 2000 to 2.36 million tons in 2004, an average annual growth rate of 82.6%, with its share of domestic market demand rising from 24.47% to 52.80% over the same period. Global output grew only about 6%. From the late 1990s, state-owned and joint-venture enterprises such as TISCO, Baosteel, Baoxin and Zhangpu built out production lines through technology import and transformation, bringing process technology and equipment to internationally advanced levels and giving the industry initial scale.

Demand is not the binding constraint. By the end of 2000, of 663 cities with established administrative status, 310 had built wastewater treatment facilities, with 427 plants constructed, an annual treatment volume of 11.36 billion m3 and a treatment rate of only 34.23%. Capital sets the pace: the scale of investment determines the scale of treatment, and the growth rate of that capital determines how fast the technology advances. Schemes with advanced technology and low treatment cost usually need more money up front, so faster capital growth means faster deployment.

Municipal treatment also needs subsidy. Price-demand elasticity is low and operation is a government monopoly, so fee setting has to respect what residents can afford. The service is public in nature and many facilities cannot be metered, so charges cannot function as market exchange and become part of public consumption instead. And the returns are broadly social, so social benefit ranks first when investment is assessed.

Processes That Fit the Load

Biological Aerated Filter

Packing is placed in a biological filter unit and oxygen is supplied artificially so large numbers of microorganisms grow on the packing. The equipment comprises 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. High sludge concentration in the reactor and compact facilities save footprint and cut reaction time.

Phosphorus Removal

Eutrophication is driven mainly by ammonia nitrogen and phosphorus discharged into water bodies, with phosphorus the more important factor, and phosphorus removal has long troubled plant operation. Traditional physico-chemical phosphorus removal consumes large amounts of chemicals, runs at high cost and makes a lot of sludge; pre-anoxic biological phosphorus removal is cheap but relies entirely on microbial uptake and release, so it struggles to meet national process requirements - harder still when reuse is on the table. The NEPA set relatively strict phosphorus discharge standards to control it. Enhanced biological phosphorus removal integrates the chemical and biological routes: anaerobic digestion of the activated sludge produces volatile organic acids that serve as substrate and nutrient for phosphorus-accumulating organisms, letting them proliferate selectively and return to the biological system so it runs in a high-efficiency phosphorus-removal state, while the phosphorus released by sludge under anaerobic conditions is eliminated chemically.

CCAS

The CCAS process - Continuous Cycle Aeration System - is a continuous-feed SBR aeration system improved from SBR (Sequencing Batch Reactor). SBR was developed as early as 1914 but was hard to apply in large plants because manual operation was cumbersome, monitoring was backward and diffusers clogged easily, so 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, which made intermittent treatment practical at scale. 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 recognised CCAS as an Innovative/Alternative (I/A) technology, making it the most advanced computer-controlled biological phosphorus- and nitrogen-removal process.

Pretreatment is minimal - only a 15 mm gap mechanical bar screen and a grit chamber. Sewage enters the pre-reaction zone at the front of the reactor continuously, where most soluble BOD is adsorbed by activated-sludge microorganisms, 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, with each step's duration and equipment operation following a pre-set, adjustable program under centralised computer control.

SPR

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 colour from the sewage, then uses micro-physical adsorption to coagulate various colloids and suspended particles into large dense flocs. Its self-designed SPR high-turbidity purifier relies on swirl and filtration hydrodynamics to separate flocs from water rapidly. Clear water passes through a self-formed dense suspended-sludge layer inside the tank, reaching tertiary treatment level and enabling reuse; sludge concentrates in the thickening chamber and is discharged periodically under pressure, with low moisture content and good dewaterability - the cake can go to mechanical dewatering and then be made into sidewalk tiles, avoiding secondary pollution.

Small Towns and Rotating Contact Oxidation

China's small towns have dispersed settlements and many low-volume pollution sources, so town-level plants are mostly below 10,000 t/d. Conventional activated sludge, A2/O, SBR and oxidation ditch carry operating costs those towns cannot sustain and fail to run normally. Rotating contact oxidation is a new generation of aerobic biofilm treatment developed from the rotating biological contactor, combining the advantages of biological contact oxidation. In the whole system the rotating shaft is the only moving part, so once the machine fails ordinary mechanics can repair it. The biomass auto-compensates with organic load: when organic matter in the sewage increases the microorganisms increase, and vice versa. Power use runs one-eighth to one-third of other aerated systems and footprint about half of conventional activated sludge.

Cyclic Intermittent Aeration

China's economic levels vary widely by region and cities with lagging economies cannot devote much funding to pollution control. Some used primary or enhanced-primary treatment whose effluent failed the national secondary discharge standard for organic pollutant removal. Cyclic intermittent aeration combines the high-load efficiency of oxidation ditches with the good effluent of sequencing batch activated sludge, meets the national primary discharge standard for organic pollutant removal, and runs about 30% below typical secondary biological systems on investment and operating cost.

Fees and the Road Ahead

Regulation and the fee system matter as much as the process. After a price hearing on adjusting treatment fees and with municipal government approval, Guangzhou finalised its reform: residential sewage is charged on tiered metering linked to Guangzhou Water's residential tiered metering - up to 22 t per household per month at 0.9 yuan/t, the 22-30 t portion at 1.2 yuan/t, and the portion above 30 t at 1.5 yuan/t. Until the policy framework, the regulatory system and the fee structure all line up, the gap between built capacity and actually treated volume stays open.