China is experiencing a rapid socioeconomic development and water pollution is a major problem in many parts of the country. This report describes a preliminary design of a wastewater treatment plant for the Zhengding City, which is near Beijing and has a population 440,000. The plant is designed with a capacity to treat wastewater of 66, 000 m3/day (a safety factor 1.2 is added to mitigate any possible future expansion demand). The plant consists of preliminary treatment, primary treatment and secondary treatment processes. The sludge treatment will be based on composting to provide fertilisers for agriculture. Detailed calculations are provided in the report to justify the design parameters and to make sure the effluent quality could meet the required demand.
Wastewater treatment plants play an important role in modern society. Wastewater treatment is the process of removing contaminants from wastewater and household sewage, which includes physical, chemical, and biological processes to remove physical, chemical and biological contaminants. Its objective is to produce an environmentally-safe fluid waste stream (or treated effluent) and a solid waste (or treated sludge) suitable for disposal or reuse (Khopkar, 2004).
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This study is to select a large urban area, design a wastewater treatment plant which will adequately treat the expected wastewater to meet the following final effluent consent parameters:
Table 1 Design requirements
The choices of each process unit should be justified with suitable sizes where appropriate. The operation of the plant including the maintenance and monitoring (both assess and process) should be outlined.
2. Urban site information
The urban area chosen in this study is Zhengding City (Figure 1) that next to my home city Shijiazhuang, China. The city has a population of 440,000. It is located in the middle of Hebei Province. The city is fairly flat so it is relatively easy to collect the wastewater to a treatment plant. The terrain map and satellite image of the city are presented in Figure 2 and Figure 3.
Figure 1 Location of Zhengding City (Modified from Wikipedia 2011)
Figure 2 Terrain map of Zhengding City (Google maps, 2011)
Figure 3 Satellite image of Zhengding City (Google maps, 2011)
The city has a continental monsoon climate with four distinct seasons. The climate is represented by the yearly average figures with: 12.7 Â°C for the temperature, 62% for the humidity, 570 mm for the precipitation, and 2,736 hours for the sunshine time, and the frost-free period exceeds 200 days per year. (Wikipedia, 2011).
3. Basic treatment processes
Wastewater treatment usually includes four stages, i.e., preliminary, primary, secondary and tertiary (Figure 4).
Figure 4 Process Flow Diagram for a typical large-scale treatment plant (Wikipedia 2011)
1) Preliminary treatment is to removes materials that can be easily collected from the raw wastewater before they damage or clog other subsequent treatment processes (trash, tree branches, leaves, etc). It involves flow handling, screening and grit removal. Preliminary treatment plays an important role in wastewater treatment plant; it can ensure normal operation of the wastewater treatment plant.
2) Primary treatment involves temporarily holding the wastewater in a tank (called primary sedimentation tanks or primary clarifiers) where heavy solids can settle to the bottom while oil, grease and lighter solids float to the surface. The settled sludge and floating grease and oil are removed and the remaining liquid may be passed to secondary treatment.
The dimensions of the tank should be designed to effect removal of a high percentage of the floatables and sludge. A typical sedimentation tank may remove from 50 to 70 percent of suspended solids, and from 25 to 45 percent of biochemical oxygen demand (BOD) from the sewage (Harrison, 2011).
3) Secondary treatment is to significantly degrade the biological content of the wastewater. The biological content is mainly from human waste, food waste, soaps and detergent. The most treatment plants use aerobic biological processes which require oxygen, food and space. The bacteria and protozoa consume biodegradable soluble organic contaminants and bind much of the less soluble fractions into floc. Secondary treatment includes activated sludge, surface-aerated basin, constructed wetland, filter bed, soil bio-technology, biological aerated filter, rotating biological contactor, membrane bioreactor, secondary sedimentation (Harrison, 2011; Wikipedia, 2011).
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4) Tertiary treatment is to provide a final treatment stage to raise the effluent quality before it is discharged to the receiving environment and includes any treatment process following conventional secondary treatment. More than one tertiary treatment process may be used at any treatment plant. Treated water is sometimes disinfected chemically or physically. If it is sufficiently clean, it can also be used for groundwater recharge or agricultural purposes. Tertiary treatment includes filtration, lagooning, nutrient removal (nitrogen, phosphorus), disinfection, odour control, etc.
4. Treatment plant design
Based on the city population of 440,000 and daily wastewater 150 litre/person, the quantity of total wastewater is estimated as
Considering possible population increase, a safety factor of 1.20 is added for the design wastewater flow rate. The wastewater quality indicators from the Zhengding urban area are presented in Table 2.
Table 2 Raw wastewater quality
4.1 Preliminary treatment design
The wastewater discharge is converted into metric unit
For the design value, a safety factor of 1.20 is included.
The suitable wastewater velocity in the preliminary treatment channel should be around 0.4 - 0.8 m/s (higher velocity should be used if the wastewater particles are large to prevent sedimentation) and the velocity passing the screen should be around 0.6 - 1.0m/s (Wang, 1999). The velocity passing through the screen should not be too large (difficult to hold the soft materials) or too small (large solid particle may settle before the screen to cause blockage). In this study, the velocity through the screen 0.7m/s, the screen gap is , water depth is
The number of gaps
The screen bars are designed with 10mm steel rod.
The width of the channel is
The quantity of the materials collected by the screen is proportional to the wastewater flow. From Wang (1999), for a medium screen, the collected materials are , so the daily collected materials are
Since the materials collected is greater than , a mechanical removing device is needed.
4.2 Primary treatment
The design is focused on the dimension of the primary sedimentation tank. Sedimentation tanks may function either intermittently or continuously. The intermittent tanks (also called quiescent type tanks) are those which store water for a certain period and keep it in complete rest. In a continuous flow type tank, the flow velocity is only reduced and the water is not brought to complete rest as is done in an intermittent type. Settling basins may be either long rectangular or circular in plan (IIT-KANPUR, 2011).
In this study, long narrow rectangular tanks with horizontal flow are used because they are generally preferred to the circular tanks with radial or spiral flow due to their more stable hydraulic flow regime. This is especially evident for large wastewater flows such as the case in this study.
A typical long rectangular tank has length ranging from 2 to 4 times their width. The bottom is slightly sloped to facilitate sludge scraping. A slow moving mechanical sludge scraper continuously pulls the settled material into a sludge hopper from where it is pumped out periodically (Figure 5).
Figure 5 A typical rectangular sedimentation tank (IIT-KANPUR, 2011)
Considering the wastewater flow volume, two tanks are designed. Design flow is , resident time in the tank is t= 2minutes=120 seconds (the normal range is 1 ~ 3 minutes), velocity is set at v=0.1m/s (the normal range is 0.06 ~ 0.12 m/s), the tank depth is (the normal range is 2 ~ 3 m) (Wang, H.C, 1999).
The total volume of the tanks
For each tank, the volume would be
The surface area of each tank
The length of the tank is
The width of the tank
Therefore, each tank is 1.8 wide, 12 m long and 2.5m deep.
4.3 Secondary treatment
For the secondary treatment, since there is sufficient land available for the treatment plant, oxidation ditch approach is adopted in this design. Oxidation ditches are relatively easy to maintain and resilient to shock loads. Fundamentally, oxidation ditch is an extended aeration activated sludge process (Figure 6). An oxidation ditch is a large holding tank in a continuous ditch with an oval shape. The oxidation ditch effluent is settled in a secondary sedimentation tank and part of the settled sludge is returned. The overview of a typical oxidation ditch is shown in Figure 7.
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Figure 6 Oxidation ditch flow diagram (WT, 2011)
Figure 7 Oxidation ditch overview (WT, 2011)
The inflow and outflow for the oxidation ditch is represented in Table 3.
Table 3 Combined inflow and outflow quality indicators
Oxidation ditch (Carousel) design parameters are: hydraulic retention time of 6 hours (normally 6 - 36 hours) and a sludge age of 15 days (normally 12 - 20 days) (WT, 2011)
The Carrousel system is based upon the principle of oxidation ditch where primary settlement, the activated sludge process, secondary settlement and sludge mineralization can take place at the same time. The principle of the Carrousel system is based on the general oxidation ditch technology. The shape of a Carrousel basin likes a race track and has a central, vertical partition wall. Micro-organisms can break down the organic substances, phosphorus and nitrogen in wastewater when the wastewater is circulating around the channel (Water in sewage, 2011).
The quantity of sludge produced each day is
The sludge concentration in the ditch is 5kg/m3 and sludge age time is 15 days. The ditch volume is (based on the method from http://www.eptec.cn/water/shws/2006-07-17/405.html)
The depth of the fluid is 4m, the total area for the ditches is
Normally, 12 ditches are used. If each ditch is 9 metres wide (total width is 108m), the ditch length is
Therefore, the final ditches are: 12 ditches with 125m long, 9m wide and 4m deep.
Now, to satisfy the effluent requirements, the oxygen demand can be calculated as
BOD oxygen demand:
Ammonia oxygen demand:
SS oxygen demand:
The total oxygen demand is
If such amount of oxygen is pumped into the oxidation ditches, the effluent quality in SS, BOD and Ammonia would be met. In this study, the inflow phosphor is already at 2mg/l, so no further action is needed to treat it. Aeration pumps can then be selected based on the oxygen demand. The secondary sedimentation tank is needed to couple with the oxidation ditches. Due to the page limit in this report, the detailed calculation for this tank is omitted.
4.4 Tertiary treatment
After the secondary treatment, the effluent quality indicators are able to meet the required values, hence no tertiary treatment processes are planned in this study.
4.5 Sludge treatment and disposal
Coarse primary solids and secondary bio-solids accumulated in a wastewater treatment process must be treated and disposed of in a safe and effective manner. Many sludge is treated using a variety of digestion techniques, the purpose of which is to reduce the amount of organic matter and the number of disease-causing microorganisms present in the solids. The most common treatment options include anaerobic digestion, aerobic digestion, and composting (wikipedia, 2011).
Agriculture plays a very important role in China and there is a huge demand for fertilisers. In this study, composting treatment for the sludge is proposed. Basically, it is also an aerobic process that involves mixing the wastewater solids with sources of carbon such as sawdust, straw or wood chips. In the presence of oxygen, bacteria digest both the wastewater solids and the added carbon source and, in doing so, produce a large amount of heat. Both anaerobic and aerobic digestion processes can result in the destruction of disease-causing microorganisms and parasites to a sufficient level to allow the resulting digested solids to be safely applied to land used as a soil amendment material (with similar benefits to peat) or used for agriculture as a fertilizer provided that levels of toxic constituents are sufficiently low (Strathclyde, 2011).
5. Operation (Maintenance and monitoring)
After construction of a sewage treatment work, the flowing maintenance and monitoring are vital. However consequences of sewage treatment plants are often neglected because of they are typically located out of sight, and as a result, the penalties for neglecting a plant can be both dangerous and costly. Sewage Treatment systems are organic environments. To operate they rely upon components such as pumps, scrubbers and motors all of which require regular maintenance and replacement (drainstore, 2011).
5.1 Familiar with facilities
In order to use facilities efficiently, the first this is that staff should be familiar with these facilities. Normally, the matters needing attention would be listed on instruction books, managers or staff need to read them one by one. Operation staff can consult manufactures about management of facilities. Training should be provided on a â€œjust in timeâ€Â basis. Staff new to technology can not absorb expert-level training; managers just need to intrigue basics to them, and then promote their acceptance of new technology later.
One issue should be attention that manufactures rarely introduce drawbacks of their products and facilities in instruction books; however every product has defects, operation staff can accumulate experience during processes of operation and observation, and know facilitiesâ€™ drawbacks gradually, so that find solutions.
5.2 Setting up sophisticated facilities profiles
The profiles of facilities mainly can be divided into three parts.
Including instruction books, drawing sheets, certification test, installation records and inspection records. These records are vital in the maintenance and operation stages, they should be stored carefully.
The second part of profile is daily operation conditions of facilities, operation staff be responsible for it. It should include operation time of each facility, their operation conditions, accumulative operation time, oil time, type of oil, quantity, reasons of malfunction and its time. Operation staffs need to write a report monthly and turn over it to higher authorities.
The last part is about maintenance, including maintenance time, cause and effect of malfunction, and solutions. After replacement, a new facility is installed, its basic information need to record, and the process of improving data quality is needed when migrating data from an old application to a new one.
According to these three parts of profiles, operation staff can analyse operation conditions and find out cause of break down. If manufacturer has legal dispute with installation unit, integrated technical profile and operation records will be good for sewage treatment company.
5.3 Setting up integrated censorship
Because many facilities of sewage treatment work are large grade, they distribute separately, and most of them in the open air, so regular inspection is needed. Central control room plays an important in inspecting and monitoring. Central control room has to work 24 hours in a day, in order to solve problems immediately.
Operation staff also need to pay attention to weather conditions that is significant for ensure facilities work normally. Operation staff can take actions to prevent facilities from damaging of disaster weathers.
5.4 Choosing optimum operation plan
As we all known, any facility has its operation life, on the premise of finishing wastewater treatment task, operation staff should avoid ineffective facilities operation, so regular maintenance work is necessary. For example:
During preliminary treatment process, if manager do not clean up screen regularly, many solid materials from wastewater would accumulate on the screens, when operation staff run a machine, these large solid materials could block and break machine. If a machine working for a long time without rest, it would increase deterioration of machine. So operation staff should justify machine operation time according flow, waster accumulation conditions, and so on, so that improve work efficiency.
Sludge Scrubber. In China, most managers make scrubbers work all around clock; actually they do not know sludge concentrations and load of scrubbers. It is demonstrated that it is more effective when scrubbers work intermittently, so operation staff can decide scrubber running time based on its flow conditions, and sewage treatment plant can save energy and electricity in this way.
5.4 Lubrication of machine
In order to machines can keep regular work, add lubrication oil to machine regularly. Zhengding is in northern part of China, four seasons are well distinguished. Lubrication oil could be frozen in winter for temperature extremely low. In summer, oil could be diluted, and leakage accidents often take place, operation staffs need to pay close attention to these circumstances.
This study describes the key design parameters for a wastewater treatment plant for the Zhengding City in China. Detailed calculations and steps are carried out to justify the design of each treatment process. Based on the quality indicators from the incoming raw wastewater and the required effluent quality, the designed plant should be able to meet the required quality demand. It should be noted that this is a preliminary design and further information is needed for a refined design (such as the population prediction for the future urban expansion, water demand from the residents, possible new industry in the area, etc). Although a safety factor of 1.20 is added in this preliminary design (i.e., the design flow is 1.2 times of the required flow based on the current wastewater flow), there are still many uncertainties that could create risks to the successful operation of the plant. The planner for wastewater treatment in this area should consider possible future expansion in the final design.