MaMunicipal Wastewater Reclamation and Reuse in Water Shorta

WANG Baozhen

* MOC expert on wastewater reuse for the joint MOC/UNDP
project on urban water Resources Optimal Management in 21st Century

* Professor, Harbin University of Civil Engineering and Architecture

1. INTRDUCTION

Although the total average annual water reserve in China is about 2,800 billion m3, and ranks sixth in the world, the average annual water reserve per capita is only 2,500m3, less than one fourth of the world average. In addition, water resources in China are distributed unevenly both in space and time. Geographically, there is more water resources in southern and eastern areas, while in northern part (north of Yantze River) accounting for 60% total territory area of China, there is only 20% of the total water reserve. [1]

With the rapid development of industry, agriculture and urban construction, as well as the progressive improvement of people‘s living standards, the demand on fresh water resources in urban areas is increasing rapidly. In mid-eighties, there were 236 cities suffered from water shortage and in nineties, of total of 644 cities in China, 450 have been facing water shortage problem.

In order to solve the water shortage problem, many cities have exploited new water sources, which are usually far apart from the respective cities. For instances, the water diversion project called "Diverting Luanhe River to Tianjin" has a canal of 234Km long and a annual flow of 1 billion m3. Another water diversion projects "Diverting Yellow River to Qingdao" consists of a 297Km long canal and a reservoirs of 140 million m3 storage capacity. Besides, about twenty more water diversion projects have been built and put into operation in some other water shortage cities.

The depletion of groundwater resource is also a critical problem facing China, A survey carried out in 1988 showed that the groundwater table in northern cities has fallen by 10-30 meter due to over pumping, and in a few cities the figure has reached 70 meters. At present in most northern cities there is no capacity for further exploitation of groundwater.

As neither surface water nor groundwater sources are available for further exploitation in many water acute shortage cities, the municipal and industrial wastewater have become most available alternative water resources, which after proper treatment can be reclaimed and reused multi-purposely, such as farmland irrigation (land treatment and utilization system), fish farming, aquatic plants growing, industrial uses like cooling water, domestic uses except drinking water, such as toilet flushing and watering of green belts, and municipal uses like impoundment of artificial lakes, watering parks and green belts and fire proofing.

2. MUNICIPAL WASTEWATER RECLAMATION AND REUSE OPTIONS AVAILABLE IN WATER SCARCE CITIES IN CHINA

2.1 Municipal wastewater reclamation and reuse in aquaculture and agriculture

The application of pretreated municipal wastewater to the fish farming and agriculture has a long history in China with successful experience. The most practicable and most widespread reclamation and reuse options are eco-pond systems and land treatment and utilization systems [2][3]

Eco-pond Systems

The application of ponds to receiving and purifying sewage and manure from nearby villages and towns while culturing fish in east and south China has been a documentarily verifiable practice with a long history of more than 2,000 years. In villages of Pearl River Delta, for example, a large number of ponds have been used to receive not only human and animal manure, but also dung of silkworms, sugar cane leaves and grass, which can be used either feeding fish directly or as nutrient sources to promote the growth of algae and zooplankton for fish farming. On the basis of the above mentioned practice and experience, various field-pond systems such as mulberry field-ponds, sugar cane field-ponds and paddy field-ponds have been developed there. The sludge removed from the bottom of ponds is usually applied to the subsoil of the field as organic fertilizer, thus forming various small ecological circulation systems.

Following up the tradition, China has now built more and more sewage and wastewater treatment ponds with eco-systems, which are different from those with symbiotic algae/bacterial systems that often result in high concentration of algae in effluents, thus causing secondary pollution to receiving waters. It is therefore necessary that this type of ponds are followed by algae removal facilities, such as micro-straining, flocculation/coagulation/settling, air flotation, rock or sand filtration units, which make the pond systems more complicated and expensive in construction, operation and maintenance, and consequently lose their inherent advantage, such as low capital and operational costs, simplicity in construction and easy to operate.

The performance of the eco-pond systems is supported by solar radiation as an initial energy source. Under the solar radiation as their initial energy source, the anaerobic, facultative and aerobic bacteria and fungi present in the ponds degrade the organic pollutants contained in the influent (wastewater) into water, carbon dioxide, ammonia, nitrite, nitrate, phosphate, etc. as end products through various intermediate degradation steps. The algae and other aquatic plants, while capturing solar energy and taking up the inorganic degradation end products mentioned above as nutrients for their growth during photosynthesis, release molecular oxygen available for aerobic assimilation. In this way, a small portion (about 2-3 percent ) of the total incident energy from solar radiation is converted into chemical form, i.e. chlorophyll and stored in aquatic plants for subsequent use by themselves and other organisms that consume them. The increased(surplus) micro-algae, bacteria and fungi serve as foodstuffs for zooplanktons, such as Paramecium, Colpidon, Amoeba, Rotaria, and Cladocera(Daphnia), which in turn serve as foodstuffs for shrimps, fish and duck. The small sized fish, shrimp, zooplankton and /or aquatic plants are consumed by duck and /or geese as available foodstuffs for their growth. The snails, clams, shrimps and filter feeding fishes also consume micro-algae, thus preventing them from overgrowth and accumulation, and maintaining good quality of effluent with low SS concentration. The schematic diagram of an ecological system is shown in Figure 1.[2]

Figure 1, Schematic Diagram of Ecological System in Eco-ponds

The eco-ponds with the help of mass and energy transfer and transformation of sub-stances present in wastewater through various food chains or webs consisting of producers (algae and other aquatic plants), decomposers (bacteria and fungi) and consumers as well as various physical and chemical processes taking place in ponds can effectively remove TSS, BOD, COD, bacteria, virus and mineral salts, while reclai-ming them in forms of aquatic plants, fish, duck, and geese as recovered resources.

In addition to hundreds thousands of ponds in rural areas that receive, utilize and treat manure and sewage from their respective nearby villages and towns by means of fish farming and aquatic plants growing, there are about several hundred municipal and industrial wastewater treatment and utilization ponds with eco-systems in operation in urban areas, which perform either solely or in combination with conventional treatment plants or irrigation farmlands.

The unit ponds applied for the treatment of sewage are classified anaerobic, facultative, aerobic, hydrophyte, fish, duck/geese and storage ones, from which various pond systems can be formed through different combination. The most important pond systems being used in China are described as follows:

In northern arid areas:
Sewage® ? preliminary treatment® facultative pond® storage ponds or lagoons (fish farming or duck geese raising in warm seasons)® irrigation on farmland® effluent;
High strength organic wastewater® anaerobic ponds-facultative ponds® storage ponds or lagoons (fish farming and duck /geese raising in warm seasons)® farmland irrigation® effluent

Figure 2 Layout of the wastewater treatment and storage lagoon in Qiqihaer City

The Qiqihaer City, on the middle reach of Nenjiang River, is situated at 123o13‘ East Longitude and 47o52‘ Latitude and in the west of Heilongjiang province. Being located in frigid temperature zone, it is characterized by continental monsoon climate with mean annual temperature of 3.2oC. In eighties, about 150,000m3 of wastewater is treated in the lagoon daily. The layout of wastewater treatment and storage lagoon in Qiqihaer City is shown in Figure 2.

The incoming wastewater have been treated at high removal efficiencies for various pollutants (see Table 6). For instance, SS and BOD were removed greater than 90%; COD, synthetic detergent (DBS) and ammonium nitrogen, greater than 80%; lignin and zinc, greater than 95%; bacteria, greater than 99%; phenol and cyanide, some 90%; and so on. A detailed removal efficiencies of the lagoon for various pollutants are shown in Table 5. It is evident form the observed results that the lagoon removed more species of pollutants at higher efficiency from wastewater in comparison with the conventional secondary treatment facilities, particularly in terms of nitrogen, COD, and refractory organic compounds. Moreover, its capital and operational costs are much cheaper than that of secondary conventional treatment facilities.

Table 6 The pollutant removal of the wastewater treatment and storage lagoon in Qiqihaer City, Heilongjiang Province. (Analytical data during the period from May to October, 1983 at water temperature from 15-20oC)

In southern areas:
Sewage or organic wastewater® preliminary treatment® facultative ponds (with floating plants like water hyacinth growth)® fish farming ponds ® duck/geese ponds® lotus or reed ponds ® effluent or
Sewage® preliminary treatment® facultative ponds (with water hyacinth growth)® fish farming in paddy fields® effluent

The pond systems, particularly large scale ones, have played increasingly important role in water pollution control in China. For example, in Changsha City, Capital of Hunan Province, about 250,000 cubic meters sewage a day, accounting for 75% of its total daily discharge, and organic wastewater both from industries and animal raising with a total flow of 50,000 cubic meters a day have been sent to many ponds for fish farming while purifying wastewater. The total area of sewage fish ponds in the city is 1430 ha. Under the conditions of sewage hydraulic loading of 400-500m3/ha/d, BOD5 loading 20-30kg/ha/d and pond water temperature 15-25℃, the ponds exhibited the following removal efficiencies: TSS, 74-83%, BOD5, 75-91%, and total nitrogen, 70%. Meanwhile, the fish production ranged from 4500 to 6000 kg/ha/an.

Yarhu pond system in the suburb of Wuhan City, Capital of Hubei Province, consisting of five ponds in series with a total area of 400 ha, receives and treats industrial wastewater from chemical industrial region of the city, with considerable high removal efficiencies for various pollutants: COD 44.6-77.2%, organic phosphorus 33.9-79.6%, P-nitrophenol 92.6-96.5%, and HCCH 56.6-87.1%.

In recent years the sewage and wastewater treatment and utilization pond systems have developed very swiftly due to their lots of merits, such as low capital and operational costs (about only one fourth to one third that of conventional secondary treatment, and even more efficient in terms of nutrients, COD and bacteria removals, and considerable income from fish farming, aquatic plants growth and/or duck/geese raising. It is anticipated that the ponds with eco-systems will become one of the major sewage treatment facilities in small cities, towns and villages.

Wetland systems

There is a huge area of natural marshes, and wetlands available for receiving and purifying sewage and /or industrial wastewater after suitable pretreatment. For instance, only in Tianjin Municipality there exists about 150,000 ha marshes with reeds, rush and cattails growth. In Heilongjiang Province the total area of natural wetlands is about 6 million ha. Therefore, there is a great potential for the development of wetlands as an appropriate wastewater treatment technology in China. In fact, many cities and towns have already employed their nearby wetlands to receive and treat municipal and industrial wastewater. For example, in Shenzhen City, Guangdong Province a 140 ha of constructive wetland system has been built and put into operation for treating sewage. In Rongcheng City, Shandong Province, 200 ha of reed strand have been used to receive and treat sewage and pulp and paper mill effluent. In Inner Mongolian Autonomous Region and Heilongjiang Province, some pulp and paper mills and other factories are also used wetlands to treat their wastewater. From 1985 to 1990 Beijing Municipal Institute for Environmental Protection studied the special topic on using surface flow wetland system (SFW) for treating municipal and paper and pulp mill wastewater. The treatment results on SFW and infiltration wetland are presented in Table 7 and the annual average treatment efficiency are shown in Table 8. [4]

Table 7 The relevant parameters on reed bed surface in Bejing and Tianjin districts

Item Tianjin City Beijing, Chanping County Beijing, Qinghe District Tianjin (Infiltration Type) Sewage Type Municipal Sewage Municipal Sewage Paper-making Effluent Municipal Sewage Treatment Capacity(m³/d) 200 500 120 1800 Pretreatment Preliminary Settling Preliminary Settling Stable pond +overland flow Preliminary Setting Wetland technological Characteristics Reed wetland +aquatic plant pond+fishpond Reed wetland+ fish pond Reed wetland+ fish pond Reed wetland+ fish pond Wetland Hydraulic load(cm/d) 1.5-3.6 4.7 1.0-1.8 6.0 Slope length (m) 130 100 - 150 Hydraulic retention time(d) 1.5-3.0 7.3 5.6 10 Reed yield(t/ha) 8.5 29.4 15.0 -

Table 8 The treatment efficiency on reed bed wetland systems(mg/l)

Site Water quality BOD₅ SS TN TP Tianjin City Influent
Effluent
Removal
(%) 120
18.1
84.9 129.5
19.5
79.9 39.56
19.54
50.6 3.30
0.98
70.3 Beijing, Changping county Influent
Effluent
Removal
(%) 125
17.8
85.8 275
17.0
93.8 14.4
5.10
64.6 0.94
0.42
55.1 Beijing Qinghe District Influent
Effluent
Removal
(%) 9.0
5.6
37.7 38.2
5.1
83.8 4.35
3.08
29.2 0.74
0.34
53.9 Tianjin City (infiltration Type) Influent
Effluent
Removal
(%) 154.7
10.3
85.0 136.7
11.4
90.0 37.6
6.15
83.4 1.43
0.32
86.0

It has been found from our investigations that reed wetlands can effectively and efficiently treat wastewater from pulp and paper mills, which is characterized by high alkalinity and high salt content. Under the conditions of HRT of 7 days and hydraulic loading rate of 2-4 cm/d, TSS and COD were removed by 75-88%, BOD5 85-95%, heavy metals and arsenic 70-90%, phenol and cyanide 80-95%, sulphate and chloride 60-70%.

A full scale or pilot scale study on four types of wetlands, i.e. constructive, natural and free water surface wetlands and reed root bed, carried out by Tianjin Municipal Environmental Protection Research Institute has shown that all the four kinds of wetlands exhibited quite high removal efficiencies for various pollutants, e.g, BOD5 87.4-92.5%, COD 72-81.7%/63. 1-82.0%, and TN 61.6-82.4%/40.2-84.1% (in warm seasons/in cold seasons respectively).

Land Treatment System

A wide variety of pollutants and nutrients can be removed very efficiently from domestic sewage and wastewater when applied on the farmland through various mechanisms, such as filtration, adsorption, ion exchange, chemical reaction, microbial metabolism in soil media and uptake by plant root system. The typical removal efficiencies for various pollutants were reported as follows: SS, BOD, total phosphorus and bacteria, 95%; trace metals, phenol, and cyanide, 90%; COD, total nitrogen and potassium, 80%. The high quality effluents from the irrigation fields can be used to recharge sharply falling water table or to improve surface water quality.

Meanwhile, the majority of sewage irrigation projects have improved the soil quality and structure as a result of an increase in organic matter, nutrients and moisture in soil, and their yields have increased markedly. According to investigations and statistics, more than 90% of the total sewage irrigation farmlands throughout the country has increased their yield. By applying municipal sewage with BOD5 70-120mg/l, T-N 15-20mg/l, NH3-N 5-10mg/l and T-P 0.5-3mg/l under hydraulic loading of 3000 to 7500 m3/ha/an, the yields of wheat and rice have increased by 800-4000kg/ha. On an average, one cubic meter of sewage applied on farmland can increase grain production by 0.5kg. The detail result of land treatment employed in the ecological municipal wastewater treatment and utilization system in Jiaozhou City, Shandong Province is shown in Table 9. [5]

Table 9 Removal of pollutants by soil-plant purification

Parameter COD_Cr Mg/l BOD₅ Mg/l TSS Mg /l TKN MG/L TP Mg/l Samples Infl. Eff. Infl. Eff. Infl. Eff. Infl. Eff. Infl. Eff. Wheatland 180
250 10
15 52
80 3
3 150 108 5
5 9.15 17.1 0.72 0.98 3.73
2.75 0.135 0.205 Maize land 148
160 10
10 45
56 3
3 87
98 5
5 15.8 10.6 0.83 0.81 2.22
2.31 0.345 0.13 Average removal(%) 184.5 11.25
93.9 58.25 3
94.8 110.75 5
95.4 13.16 0.835
93.6 2.75 0.20
92.7

On the other hand, however, there is a little portion of wastewater irrigation projects (less than 10% of its total area) has been deteriorated by irrigation with toxic and harmful industrial wastewater, thus causing yields decrease and pollution of crops as well as soil mantles and groundwater. Moreover, the vast majority of sewage irrigation systems lack necessary pretreatment facilities and waterproof measures. As a result, the agricultural produce, mantles and groundwater have been polluted to some extent.

As sewage irrigation projects can receive and utilize while treating sewage only seasonally, i.e. during the crop-planting period. In the remainder of a year, the sewage or wastewater has to be discharged without any treatment into receiving waters, thus causing pollution to them. Therefore, some efforts have been made to transform some seasonal sewage irrigation projects into all the year round land treatment and utilization systems, which generally consists of preliminary treatment, stabilization ponds, storage lagoons and irrigation fields.

2.2 Municipal Wastewater Reclamation For Domestic Water Uses Except Drinking Water

The effluent from the secondary municipal wastewater treatment plant and the effluent from the lightly polluted domestic wastewater or grey water treatment facilities serving a building, a group of buildings or a residential sub-district are sent into the dual water systems for multipurpose uses such as watering of gardens, parks and green belts, washing floor and car, and toilet flushing.

At present in Shenzhen City, Guangdong Province, Qingdao City Shandong Province, Dalian City, liaoning Province. Beijing, Capital of China and some other water scarce cities in China have built some such kind of wastewater reclamation and reuse plants and put into operation, which reserve the water resource capacity in the respective cities three to five percent.

2.3 Municipal Wastewater Reclamation and Reuse in Industry

The effluent from some secondary municipal wastewater after further proper treatment like filtration and disinfection is applied in some industries for various uses such as cooling water, process water, hydraulic transportation of slag and ash and injection to the oil field for oil exploitation.

The effluent from Chunliu River WWTP employing conventional activated sludge process in Dalian City after filtration is sent to a nearby chemicals factory with capacity of 10000m3/d as cooling water. The performance of the wastewater reclamation plant is quite good and well meets the standard of cooling water.

The effluent from Chengfeng Zhuang WWTP employing A/O activated sludge process in Daqing City designed by the author and his colleagues after filtration through fibrous filters is injected into the oil bearing underground layers for oil exploitation. The treatment capacity of wastewater treatment and reclamation plant is 30,000m3/d, which performs very well with final effluent well meeting the standard of recharge water for oil exploitation.

2.4 Municipal Wastewater Reclamation and Reuse for Recreation

The effluent from some municipal wastewater treatment plants is discharged into artificial lakes or creaks to impound this water bodies for recreation such as fishing, boating or swimming if the water quality meets the standards of swimming water.

The Wenchang WWTP in Harbin City designed and operated for the discharge of its effluent into the nearby Majia Creak to improve its water quality, which will improve the surrounding environment greatly and make the Majia Creak become a recreational spot.

The ecological municipal wastewater reclamation and reuse system treating municipal wastewater from Xicheng district, Dongying City, Shandong Province designed by the author and his colleagues is now under construction, which is to be put into operation by the end of 1999. The effluent from this system will be discharge to the Guangli River for augmentation of the river flow and the improvement of river quality. As a result, the Guangli River will be improved significantly in terms of aquatic environment.

2.5 Municipal Wastewater Reclamation to Recharge Groundwater

At present, in many water scarce cities in China, in northern areas in particular, the groundwater table has been dropped sharply because of the over exploitation. For example, in Shijiazhuang City the water table has been dropped from 3-5 meter in 50s to 35-45 meters in 90s due to over-pumping of the groundwater as the main water source, which has made the city use surface water from two big reservoirs as the main water source instead of groundwater since 1996.
The farmland irrigation using reclaimed municipal wastewater has indicated that it is an effective measure to rise water table through slow infiltration of the effluent from the ecological wastewater treatment and utilization system in Jiaozhou City, Shandong Province that consist of preliminary treatment and primary treatment (settling basin with inclined plates), anaerobic pond, facultative pond, fish pond and wetland, from which the effluent is applied on the nearby farmland growing wheat, rice and corn with a total land area of 1000ha.The system put into operation since 1993. Up to now, the ground water table has been raised 2-3 meters and recovered to the original ones, which has been effective to prevent the seawater from invasion to the fresh water aquifers.

3. THE PROPOSED MUNICIPAL WASTEWATER RECLAMATION AND REUSE OPTIONS IN SHIJIAZHUANG CITY

Shijiazhuang City facing with acute water shortage and the situation is becoming from bad to worse with years to come considering the rapid economic development, accompanied by the increasing water demand and limitation of new water source exploitation, the municipal wastewater reclamation and reuse has been becoming an increasingly important and stable alternative water resource in the city.

There is only one municipal wastewater treatment plant called Qiaoxi WWTP at present in the city with a total treatment capacity of 200,000m3/d, there are two other WWTPs called Qiaodong WWTP with a treatment capacity of 50,000m3/d and WWTP in the Economical Development Zone with a treatment capacity of 100,000m3/d in design. However, the effluent from the existing Qiaoxi WWTP has not been utilized effectively by planning and most of it is discharged into the nearby receiving waters. In fact, there is a large area of farmland in acute water shortage and many farmers have used the effluent spontaneously to irrigate their farmland, and have good harvest due to moisture and fertilizer improvement from the effluent.

If all the effluent from Qiaoxi WWTP and the other two WWTPs to be built and put into operation in the near future are reused in agriculture, industry and municipal and domesticj purposes, the water resource management and conservation will be improved substantially, which will either release the water shortage or improve the water environment remarkably. Therefore, the following wastewater reclamation and reuse options are proposed for Shijiazhuang City:

* All the effluent from the exiting WWTP and two other WWTPs to be built in the future should be reused in various aspects by proper planning, of which the main reuse is in agriculture. It is suggested to design and construct farmland irrigation system. If all the effluent from these WWTPs is applied on the farmland for irrigation, the total irrigation farmland areas is about 60,000ha or 900,000mu(1ha=15mu), which is estimated according to hydraulic load of 5000m3/ha.an. After these application the farmland yield of wheat and corns could increase by 10-15 percent, or the total increase of crop production is about 45,000-50,000 ton per year, which is sufficient to sustain life of 100,000 people or more.

Besides, the effluent applied on the farmland is further purified very efficiently through various mechanisms, such as filtration through soil medium, biological degradation and assimilation by microorganism present in soil medium, physical and chemical processes taking place in the soil medium and the uptake of nutrients and other constitutes of effluent by root system of plants growing on the farmland. The good quality of filtrate will infiltrate downwards to the groundwater, which will cause the water table rise gradually and augment the ground water as a stable water source. As the purified municipal wastewater could meet the 2-3rd class surface source water quality standards in our experience from case studies of some full-scale pond-land irrigation systems existing in Jiaozhou City, Shandong Province; Qiqihar City, Heilongjiang Province and Jining city, Inner-Mongolian Autonomous Region, the industries could use the groundwater as source water for various uses in their production processes. This will be much more convenient and cost effective compared with direct reuse of municipal wastewater through short water recycling, which need a complicated and expensive advanced treatment to make the final product water meet the standards of respective reused water.

It is preferable to set up a dynamic balance between the pumped flow from groundwater for industrial, domestic and municipal uses and recharge flow into the groundwater, from farmland irrigation areas or slow infiltration systems and maintain a reasonable and stable water table, say 10-15meters under ground surface. The schematic diagram of the proposed storage/polishing pond- farmland irrigation system is shown in Figure 3.

Figure 3 Large scale and range water recycling cycle through storage pond-farmland irrigation system
proposed to build in Shijiazhuang and other water scarce cities in China.

*. In order to conduct all the year-round treatment and utilization, it is suggested that the storage pond or lagoon system be constructed before the irrigation systems. The storage capacity of the pond system should be large enough to store all the effluent flow in the non-irrigation period.

The storage pond system will perform multi-functionally, in which the influent will be further purified through various mechanisms, such as physical, chemical and biological processes, and the purified water will partially infiltrate downwards into ground to recharge groundwater on the bottom of the pond system with no water proof liner. In other words, the storage pond system and the farmland irrigation system will jointly operate both as all the year-round farmland irrigation system and groundwater recharge one. As a result, a large scale water recycle will be realized, which is much cost-effective compared with the short or small water recycling systems commonly practiced at present, such as dual water system, direct reuse in industries through complicated advanced treatment of the effluent from the WWTPs.

3. When the polishing/storage ponds-irrigation system is put into operation, the groundwater table will rise gradually due to the recharge of the effluent through slow infiltration

4. CASE STUDIES ON MUNICIPAL WASTEWATER RECLAMATION AND REUSE IN SOME WATER SCARCE CITIES

4.1 Babaishang Wastewater Reclamation Plant in Daqing City, Heilongjiang

The author has been as principal consultant in charge of the feasibility study and design of the Babaishang Municipal Wastewater Reclamation Plant with a capacity of 50,000m3/d serving 250,000 P.E. in four residential sub-districts in Daqing city, The final effluent will be injected into oil bearing underground layers for oil exploitation, and thus should meet the standard of the injected water quality under high pressure injection, such as TSS￡3mg/l, BOD5￡5mg/l and DO￡0.5mg/l.

The raw municipal wastewater to be sent to the WWRP is characterized by low concentration, with the typical BOD5 70-100mg/l, COD 200-250mg/l, TN 25-30mg/l, NH4-N 15-20mg/l, TP 3-5mg/l.

The treatment flowchart is designed as follows:

The mechanically cleaning screens are used for removing coarse solid wastes for preventing the postpositional treatment units from clogging; and the rotating type of grit chambers for removing inorganic solids. The equalization basin with HRT of 4 hours is used for equalizing wastewater flow and quality. The four sets of hybrid bio-reactors operating in parallel are the combination of activated sludge and submerged biofilm processes, which is more efficient to treat low strength wastewater, in which the HRT is 4 hours, the total MLSS 1500-2000mg/l, and air to water volumetric ratio is (3-4):1. The four circular final clarifiers with a diameter of 30m equipped with 18 perforated radial effluent collecting pipes and a rotatory scrapper for each one set, from which the effluent is pumped to the eight dual-media filters with a diameter of 4.5 meters. The filtrate is disinfected in the contact basin with HRT of 30min and then to the clean water storage basin with HRT of 4 hours, from which it was pumped to the injection pumping station in the oil field.[6]

The layout and treatment elevated flowsheet are shown is Figures 4 and 5 respectively.

The design of the project will be completed in October 1999 and its construction completed by the end of 2000, when it will be put into operation.

4.2 Ecological Wastewater Treatment and Utilization System in Dongying City, Shandong Province

Dongying City located at the Estuary of yellow River, where exists the Shengli (Victory) Oil Filed, the second large Oil Field in China with an annual crude oil production of 32 million tons, is facing with an increasing acute water shortage because of the sharp decrease of flow in Yellow River with the zero flow period over 200 days in 1996 and 1997 respectively. As a result, a large area of the existing farmland has been abandoned due to water shortage for irrigation and aqua-culture.

Therefore, the municipal wastewater has to be reclaimed and reused as a regenerated or renovated water resource. The author and his colleagues at the invitation of the municipal government have been in charge of feasibility study and design of the municipal wastewater treatment plant. After many times field surveys, it is finally decided that a multipurpose farm that has been abandoned for several years because of no water flow from Yellow River, even though the hydraulic engineering facilities originally designed and constructed for use of Yellow River water are still in good condition. Therefore, if the municipal wastewater is diverted into this farm, after proper treatment in the existing hydraulic engineering units like a reservoir with a storage capacity of 1.5millions of water and surface area of 36ha with modification into treatment units, such as advanced hybrid anaerobic ponds, aerated ponds and aerated fish ponds in series, the effluent will be good enough in quality and reclaimed and reused for fish farming, lotus, reed and rice growing.

With this concept, the author and his co-workers has designed the ecological wastewater utilization and reuse system (EWTUS) in this farm with the treatment and utilization flowchart as follows:[7]

Treatment Units
Advanced hybrid anaerobic ponds(AHAP)

The AHAP developed by the author is the combination of advanced anaerobic pond with digested pits at bottom developed by Prof. W.J. Oswald with the intensified pond packed with biofilm carrier developed by the author, which is very effective to treat organic wastewaters with a typical COD and BOD removal of 85% and 90% for high strength wastewater and 60-70% for municipal wastewater at a temperature of 20oC or higher. Therefore, the AHAPs are employed for removing 40-50% organic load with the following design parameters: HRT 1.5d; effective water depth in the ponds 4m, the depth of digestion pits 1.0m.

The submerged biofilm carrier package is to be placed 30cm below water surface with a depth of 1.0m. Under the solar radiation, the surface of the carriers will be covered by algae and bacteria. Through photosynthesis and algae/bacterial symbiotic effect, which can oxidize and degrade sulfur containing odorous compounds like H2S and mercaptan and control odor effectively.

Aerated ponds(AP)

The effluent from the AHAPs or the influent of the AP contains various organic intermediates from anaerobic degradation of organic substrates originally present in the raw wastewater, which can be easily degraded into final products of CO2, H2O and some other inorganic salts in the aerated ponds under intensive aerobic conditions.

The HRT is designed to be 1.5d, the specific aeration power is 1w/m3. 16 HEX-1 high efficiency surface aerators (3kw) are to be installed in the four cells of AP, which will supplying sufficient oxygen for biodegradation in the APs.

Aerated Fish Ponds(AFP)

The influent of the AFPs or effluent from the aeration ponds is good enough to farm fish. Various species of fish such as silver, big head, cruisian, ordinary and red carps are to be farmed in the AFPs. The total HRT is about 12 days, the effective water depth 3.6m, and the total water surface area about 30ha.

Fish Ponds(FP)

With the utilization of solar energy under solar radiation, various food chains are formed such as : bacteria/algaezooplankton like daphnia-fish; micro-algaefiltered feeding fish; aquatic plants and macro-algae? grass feeding fish. According to the mentioned different food chains, some different species of fish will be farmed. As a result, the influent will be further purified through the fish farming.

Lotus Ponds (LP)

The effluent of the FP or the influent of the LP contains some fish excreta, which can be removed in the LP through sedimentation. The organic sediment will serve as organic fertilizer to promote the growth of the lotus, which will very beautiful to see when its flowers bloom. Besides, its roots and seeds are very delicious to eat as a valuable vegetable or food. The influent will be further purified through various mechanism in the ponds.

Reed Ponds(RP)

It has been indicated from our researches that the reed ponds or wetland are very efficient to remove various pollutants form wastewater, such as heavy metals, organic pollutants, pathogenic microorganisms and inorganic salts with quite high removal efficiencies. Under the conditions of HRT of 3-7days, hydraulic loading rate of 13-5cm/d, and water temperature of 15-20oC, the typical removal of Hg. Cd, and Cu are 80-95%; phenol and cyanide 80-90%; total bacteria 99.9%; total coliform >99%.

The design parameters are described as follows:
- total water surface area 37ha
- Effective water depth 0.5m
- HRT=1.86

The effluent from the reed wetland will reach the following water quality parameters: BOD5 5-10mg/l, TSS 5-10mg/l, COD 30-50 mg/l, TN 5-10mg/l, NH4-N 3-5mg/l, TP 1-2mg/l, and TDS ￡1000mg/l, which is good enough for farmland irrigation.

The design of the EWTUS in Dongying City has been completed recently and the construction will be completed in September 1999, and the EWTUS will put into operation in early October 1999.

The total capital cost of this project is 68 million Yuan(RMB), or US$ 8.2million,and the unit capital cost is 680Yuan or US$ 82; (m3/d), which is about one third that of conventional treatment plant employing activated sludge process.

The unit operation /maintenance(O/M) cost is only 0.1 Yuan/m3 or US$ 0.012/m3, in consideration of the significant income from fish farming. Lotus and reed planting, which could compensate most of the O/M cost.

Reference

[1] B.Z.Wang and Z.J.Xhu, "The Water Resource and its Pollution Control in China", proceedings of Water Reuse Symposium IV, Denver, Colorado, USA, pp.65-76(1987);
[2] Baozhen Wang, W.Dong and Q.L.Zhao, "Eco-pond Systems for Wastewater Treatment and Utilization in China", Proceedings of 3rd IAWQ International Conf. on Appropriate Waste Management Technologies for Developing Countries. Nagpur, India, 25-26 February, 1995, Vol. 1. pp. 415-430, 1995;
[3] 73B. Wang, A. Dai, D.Wan, Q. Zhao and X. Cao, "Case Studies on Pond-land Systems Treating Municipal Wastewater in the Northern Areas of China", Proceedings of Water Environmental Federation, 68th Annual Conf. & Expo. Miami Beach, Florida, U.S.A,21-october, 1995Vol.4.pp.511-522, 1995;
[4] Baozhen Wang, "Status and Trend of Appropriate Technologies for Waste Treatment, Utilization and Disposal in China", Proceedings of IAWQ International Conference on Appropriate Waste Management Technologies, 27-28 Nov. 1991;pp.1-14 Murdoch University Perth, Australia (1991);
[5] B.Z. Wang, A.L. Dai, D.R. Wang and Q.L. Zhao, "Performance of a Full Scale Pond-land Treatment System", Proceedings of 4th IAWQ International Conference on Wetland Systems for Water Pollution Control, Guangzhou, China, 6-10 Nov. 1994,pp.549-552, 1994;
[6] Baozhen Wang and Lin Wang, "Feasibility study on the construction of Babaishang Sewage Treatment and Reclamation Plant", Water Pollution control Research center, Harbin University of Architecture and Civil Engineering, September, 1997(in chinese);
[7] Baozhen Wang and Lin Wang,"Feasibility study on the construction of Ecological wastewater Treatment and utilization system in Dongying city, Shandong province", October 1997.(in Chinese)