Environmental Water Treatment Knowledge: The Meaning and Role of Wastewater Reuse

In industry, technologies such as electrochemical descaling, membrane concentration and co-treatment of wastewater (e.g., using brewery wastewater as a biomass carbon source for wastewater treatment plants) enable graded reuse and cascade utilization of wastewater at the enterprise or park level.

Looking at the development of wastewater reuse at home and abroad, it has received fairly widespread attention, and many water-scarce regions have built a number of technically reliable, scientifically managed and stably operating reuse projects. Given the current situation at home and abroad, the development directions of wastewater reuse can be summarized as follows:

1. Establishing reasonable and complete reclaimed water quality standards. For wastewater reuse, some uncertainties to human health and the environment remain. As there is no comprehensive scientific basis yet, reclaimed water quality standards vary greatly among countries. In-depth research and the establishment of reasonable, complete reclaimed water quality standards will greatly promote the development of wastewater reuse.

2. Developing efficient and inexpensive wastewater reuse treatment technologies. Sound treatment technology is the guarantee for further development of wastewater reuse. Common treatment methods include:

(1) Solid-liquid separation: flocculation, sedimentation and filtration;

(2) Biological treatment: aerobic biological treatment, oxidation ponds and disinfection;

Wastewater process - MBFB membrane biofluidized bed

Agriculture is a major user of municipal wastewater reuse, mainly for irrigation of field crops, flowers and woodland. Reusing wastewater for farm irrigation not only provides a stable water source for agriculture; the nitrogen, phosphorus, potassium and other components in the wastewater also fertilize the soil, reducing fertilizer use and increasing crop yields, while the soil's self-purification capacity further purifies the wastewater. Reuse can also curb uncontrolled over-extraction of groundwater in rural areas. But if wastewater quality does not meet requirements, it will damage soil structure, cause pesticides and heavy metals to accumulate in crops and soil, and reduce the quality and yield of agricultural products. Limits on pollutants in reused wastewater should be based on crop type and growth stage as well as hydrogeological conditions, and the water quality must comply with the Farmland Irrigation Water Quality Standard.

Wastewater irrigation carries risks. Due to insufficient treatment or underestimation of long-term irrigation risks, China has learned many hard lessons: the Zhangshi irrigation district in Shenyang used wastewater irrigation for more than 20 years, polluting 2500 hm² of farmland with severe cadmium contamination (cadmium in rice of 5~7 mg/kg); wastewater irrigation in Tianjin's suburbs polluted 23000 hm² of farmland; in Guangzhou's suburbs, 2700 hm² of farmland was polluted by wastewater irrigation and about 13333 hm² of soil was contaminated by applying polluted bottom sludge, with the polluted area accounting for 46% of cultivated land; a sampling survey of a wastewater-irrigated district in Beijing in the mid-1980s showed that about 60% of soil and 36% of brown rice had pollution problems.

It is mainly used to supplement urban water systems and for irrigation of green belts and gardens. A city without water lacks vitality. Using reclaimed water to replenish rivers and lakes in place of other water sources kills two birds with one stone - it achieves the goals of using high-quality water for high-grade uses and saving water, while beautifying the environment. Water scarcity is a key and difficult point in Beijing's ecological construction; fully exploiting reclaimed water will provide an ample water resource guarantee for supplementing urban water systems and greening. With the construction of ecological residential areas in Beijing, urban greening water use will keep growing, and reclaimed water will become the main source of urban greening water.

Surveys show that industrial water accounts for about 25% of Beijing's total water use, so there is still great potential for savings. Facing shrinking freshwater and rising water prices, industrial enterprises - besides recycling their own wastewater to raise reuse rates - are paying increasing attention to municipal wastewater reuse. Industrial water quality requirements vary greatly by use; the higher the requirement, the higher the treatment cost. The ideal reuse targets are cooling water and low-grade process water (washing, ash flushing, dust suppression, direct cooling, etc.). When considering whether a process can use reclaimed wastewater, the required water quality must be met and the costs of reuse and treatment must be calculated for maximum economic benefit.

It is mainly used for construction, road spraying, car washing and toilet flushing. Estimates suggest that if Beijing's more than 2 million vehicles all used reclaimed water for washing, nearly a month's domestic water for 13000 households could be saved each day. Special attention should be paid to hygiene when reusing reclaimed water to avoid harming consumers' health. In addition, reclaimed water should contain no pathogenic bacteria; it should be clean, odorless and non-toxic, with suspended solids meeting application requirements.

In recent decades, sustained drought and over-extraction of groundwater have created funnel zones of more than 2500 km² in Beijing, seriously affecting the surface ecosystem and the safety of groundwater aquifers. After secondary treatment, municipal wastewater can be recharged underground; after flowing a certain distance, it is developed together with the original groundwater as a new water source. This both prevents land subsidence caused by over-extraction and uses soil self-purification to improve recharge water quality, directly supplying industrial and domestic miscellaneous water. Groundwater recharge has very high water quality requirements: before recharge, the water must undergo biological treatment (including nitrification and denitrification) and toxic organics and heavy metals must be effectively removed; otherwise the aquifer will be polluted.

(4) It should meet the water quality requirements or standards of the application object;

(5) It should be acceptable to users and the public;

(6) The reuse system should be technically feasible and easy to operate;

(7) Its price should be lower than tap water;

(3) Advanced treatment: activated carbon, air stripping, ion exchange, lime treatment, membrane processes and reverse osmosis.

The MBFB membrane biofluidized bed process for wastewater reuse builds on existing compliant discharge: through the biofluidized bed and ceramic membrane separation system, COD, NH-N, turbidity and other indicators are further reduced. The effluent can be directly reused, or serve as pretreatment for RO desalination, replacing lengthy filtration steps such as sand filtration, security filtration and ultrafiltration; the reduced organic content greatly extends RO membrane life and lowers reuse water treatment costs. The inorganic ceramic membrane separation system is the world's first inorganic membrane separation system dedicated to wastewater treatment; compared with other organic and inorganic membranes, it offers high flux, backwashability and fully automatic operation.

The membrane biofluidized bed process is based on the biofluidized bed, using powdered activated carbon (PAC) as carrier combined with the solid-liquid separation of the membrane bioreactor (MBR) process. The reactor integrates the physical adsorption of activated carbon, microbial degradation and efficient membrane separation; hard-to-degrade small-molecule organics in the water undergo full mass transfer and mixing with the fluidized powdered carbon under aeration, are adsorbed and enriched on the carbon surface, forming local pollutant concentration zones. The porous carbon surface also provides a special surface for microbial growth, adsorbing large numbers of microbial communities - especially those metabolizing the target pollutants. Powdered activated carbon also strongly adsorbs dissolved oxygen; under high dissolved oxygen, microbes oxidize and decompose the small-molecule organics enriched on the carbon surface. The ceramic membrane separation system then separates water from the powdered carbon and other suspended particles loaded with organics; through cross-flow filtration the wastewater is further purified to meet reclaimed water standards. Research shows MBFB can effectively remove ammonia nitrogen, COD and other refractory small-molecule toxic organics from slightly polluted water.

In the MBFB reaction system, powdered activated carbon (PAC), adsorbing large numbers of microbes, becomes biological activated carbon (BAC). PAC thus not only adsorbs and enriches small-molecule organic pollutants, but also adsorbs and protects microbes, adsorbs dissolved oxygen, supports microbial decomposition of small-molecule organics under locally high pollutant concentration and high dissolved oxygen, and undergoes biological regeneration. Through PAC, microbes, dissolved oxygen and pollutants, under intense fluidization, mixing, mass transfer and shear, efficient decomposition of small-molecule organics in slightly polluted water is achieved.

1. PAC adsorbs and enriches small-molecule organics: PAC enriches pollutants to form local high-concentration zones, favoring microbial growth and decomposition of small-molecule organics;

2. PAC adsorbs and protects microbes;

3. PAC adsorbs dissolved oxygen: as the carbon particle diameter decreases and specific surface area increases, PAC's adsorption of dissolved oxygen grows stronger;

4. Microbial decomposition of small-molecule organics: through PAC's adsorption and enrichment of microbes, pollutants and dissolved oxygen, and PAC's protective effect on microbes, microbes can effectively use trace organics as substrate and dissolved oxygen as electron acceptor to decompose organics in slightly polluted water, achieving deep purification;

5. Biological regeneration of PAC: the biofilm on the carbon surface oxidizes and decomposes adsorbed organics, restoring adsorption capacity through biodegradation and achieving PAC's biological regeneration. In the MBFB system, intense three-phase mass transfer, mixing, turbulence, shear and friction between carbon particles continuously slough off aged biofilm from the carbon surface, keeping MBFB's adsorption and biodegradation highly efficient.

1. The powdered activated carbon can be used long-term without replacement or regeneration;

2. Three-phase mass transfer and mixing give high reaction efficiency;

6. Pollutants are highly enriched and biomass is large;

7. Treatment of slightly polluted water is highly effective.

MBFB core - inorganic ceramic membrane

The DECLEAN inorganic ceramic membrane system developed by Seattle Environmental Technology Inc. in the US, building on ordinary ceramic membrane research, uses high-tech modification to reduce membrane fouling and greatly increase membrane flux, effectively overcoming the two biggest obstacles to applying inorganic ceramic membranes in water treatment (high price and low flux), making their application in water treatment possible.