Water reuse treatment technology and water quality standards
Secondary disinfection (100-1000 fecal coliforms/100mL) after appropriate disinfection is widely used in various applications, including limiting contact reuse (such as some irrigation methods and non-contact landscape water), environment Repair and many industrial applications. Some forms of environmental remediation require removal of nutrients and increased levels of disinfection. When recycled water is used as industrial cooling water, hardness, ammonia and dissolved solids need to be removed. It is required to have a series of treatments before being used as feed water supplement, depending on the dilution degree of the original water supply and the formation of treatment. For example, when the water is passed through the soil and then remitted into the original water source, it can be considered as a treatment. Process. At present, many water reuse projects are using membrane treatment technology to produce recycled water. One of them is to use secondary effluent as raw water, after microfiltration (pretreatment), reverse osmosis, and sometimes ultraviolet disinfection. Microfiltration membranes and ultrafiltration membranes can also be incorporated into biological treatment processes to replace traditional secondary sedimentation tanks.
1 Introduction
There are many methods for the regeneration and reuse of water and they operate normally in practice (US EPA, 1992). Each type of use has different water quality requirements and therefore requires different levels of treatment. Table 1 summarizes the commonly used methods of returning water use and the corresponding level of treatment required, from the highest level to the lowest level. This article will discuss the different ways of using and the corresponding processing methods, and will discuss some of the emerging processing techniques.
2. Agricultural water reuse for non-food crops and deep-processed food crops
Reclaimed water can irrigate various crops that humans do not directly consume, such as crops that are used to irrigate those animals (grass, grass, sorghum, corn, soybeans, etc.) and crops that require deep processing (such as wheat, perhaps There is also rice). The primary question related to public health and the environment is whether this recycling method will bring toxic substances and pathogens into our food supply. There are two main mechanisms for controlling toxic substances, including not allowing industrial wastewater (which is likely to contain toxic substances) to be discharged into the sewage collection system and sewage treatment system to be reused. To avoid industrial wastewater, we can collect sewage from the sparse service area of ​​the plant or require the factory to first remove the toxic substances from the wastewater before discharging it into the municipal drainage network. There are many ways to remove toxic substances from wastewater. In some practical applications, basic secondary processing can be adequately protected. In fact, primary processing may be sufficient, while secondary processing adds a level to toxic substances. In the secondary treatment, water is allowed to be stored back to meet the needs of different irrigation seasons. The spread of pathogenic bacteria is solved by preventing the general public from direct contact with sewage and moderate sterilization.
Water for agricultural reuse can be processed through a variety of treatments, including primary treatment, secondary (biological) treatment, and conventional sterilization processes. The biological pond treatment system is widely used for the stabilization of biodegradable organic matter, so when the seasonal storage of recycled water does not produce too many unpleasant problems (odor, germs, etc.), at the same time, the biological pond has a natural disinfection effect. The sewage treatment system, including an anaerobic or aerobic pond, is used for the basic treatment of sewage, followed by a facultative pond for further treatment and disinfection of the sewage, and also for seasonal storage. The reclaimed water that has been pretreated and sterilized can be used for agricultural irrigation according to the water requirement of the crop. Such a system allows the reuse of water and its nutrients in municipal sewage, but controls the amount of toxic substances and disinfects them. From the perspective of water management as a whole and in the long run, when water is used to irrigate crops, further treatment of the soil removes pollutants and reduces emissions of pollutants in the environment, thus reducing emissions of pollutants to surface water and groundwater. the amount. Net water use has fallen, as water previously used as agriculture can now be diverted to urban water supplies or left in the form of surface or groundwater.
3. Environmental restoration
Environmental remediation refers to the discharge of sewage into the environment with qualified water quality to maintain the flow of the water environment and maintain the function of the environment. The required treatment is based on the assimilation capacity of the contaminant environment. At least conventional secondary (biological) treatment and a certain degree of disinfection are generally required. Some environmental restorations require more advanced treatments, such as removing nutrients. Some conventional processing systems are available for these situations. At present, one of the emerging treatment processes is secondary treatment (traditional secondary treatment or biological pond secondary treatment) followed by humidification treatment. Wetlands not only provide treatment (improved water quality) but also improve the environment. Wetlands can be designed as wildlife habitats or as open green spaces in urban areas.
Palm Beach County is a humid subtropical climate. On the contrary, the Tres Rios Wetland is located near the city of Phoenix, Arizona, and is a desert area. However, there are many similarities between the Tres Rios Wetland Treatment System and the Wakodhatche Wetland Treatment System. As shown in Figure 3, they all have shallow aquatic aquatic plant growing areas, deep water areas and habitat islands. Green wetlands are a popular addition to the desert landscape of Phoenix, while also improving water quality and providing habitat for wildlife.
4. Limit contact reuse
Restricted contact reuse includes urban water reuse, which should control public access to water to protect public health. For agricultural recycling, the main risk is that such recycling may introduce toxic substances into the environment and spread diseases. Toxic substances can be controlled by reducing the concentration of toxic substances in the recycled water. Various methods can be used, such as controlling emission sources and Sewage treatment. The control of disease transmission can use certain disinfection measures, while preventing the public from directly contacting the water. Proper restrictions on contact reuse also include irrigation and landscape waters.
Limit contact re-use is generally at least secondary treatment, the health index is 100-1000 fecal coliforms / 100mL. Secondary treatment can control the amount of toxic substances in raw water (industrial wastewater or domestic sewage), and can also reduce the troubles in the water distribution system when water is reused. Incompletely sterilized return water can only ensure the safety of the system operator when it comes into contact with water occasionally, because the operator can take regular and appropriate protective measures. However, there is no guarantee that the general public will be in full contact with the safety of returning water. To protect the safety of the public, they are not allowed to come into contact with water frequently. For example, when using water refilling, the watering time is limited to when the public is not on site, or when the public is isolated from the water (drip irrigation). As another example, fishing and/or swimming in the lake is not permitted when water is used to limit the landscape water that the public is in contact with.
5. Unrestricted contact reuse
Unrestricted contact re-use is produced through a more advanced process that is safe for the public to contact (not consume). Although the specific requirements vary from one region to another, California's provisions in its “Article 22†provide a consistent solution that has been widely accepted. These require a high degree of disinfection, significantly reducing the chances of pathogens in reclaimed water. The basic treatment process is biological treatment to reduce biodegradable organic concentrations and total suspended solids (TSS).
6. Industrial recycling
Different industrial reuse methods require different reuse water quality. Secondary treatment and moderate disinfection are generally required to minimize the amount of pollutants in the return water and to protect the health of factory workers. The reuse of water as a factory cooling water is widely used. It requires the removal of hardness in the water that causes fouling of the cooling device, as well as the removal of ammonia which can cause corrosion and biofouling of the device. Figure 4 shows the process flow for this type of reuse. Secondary effluent (generally from existing wastewater treatment plants) is filtered through lime water softeners and granular media. If the secondary effluent as the raw water for reuse is not subjected to nitrification, the nitration needs to be incorporated into the filtration process. One of the emerging treatment methods for industrial recycling is the desalination treatment using a membrane device.
In particular, the secondary effluent and reverse osmosis process using microfiltration or ultrafiltration pretreatment is increasingly used in this field. Further discussion of this process will be combined with the process of replenishing the feed water.
It is then filtered to reduce the concentration of particulate matter and finally to disinfect. The removal of particulate matter can help with disinfection in several ways. First, some of the larger pathogens, such as Giardia and cryptosporidia, can be removed directly by filtration. Second, the removal of particulate matter makes the following disinfection treatment more effective. It is because after the biological treatment and filtration, the pathogenic bacteria remaining in the water are in a free state, and of course it is easier to be killed in the sterilization process. Chlorine disinfection and ultraviolet (UV) disinfection are common methods. Table 2 summarizes some of the processing requirements in order to bring the recycled water to the standard for unrestricted contact applications.
7. Supplementary water supply and emerging technologies
Supplementary water supply is a deliberate way of returning water to the drinking water supply. Return water can be used to introduce surface water (such as reservoirs) or groundwater sources for drinking water. As with the industrial recycling method, this recycling method can also adopt a variety of processing methods. The degree of treatment required is based on the degree of mixing of the recycled water and the natural water body and the processing capacity prior to being extracted for use as a public water supply.
If the proportion of recycled water injected into the groundwater through the surface flow is quite small, only secondary treatment and moderate disinfection are required. The diafiltration of the soil removes large amounts of organic matter and pathogenic bacteria from the reuse water and then mixes it with the groundwater. The main problem to consider in this case is the possibility of introducing nitrogen, which can be converted to nitrate in the soil. Natural groundwater and recycled water are mixed and some of the treatment is carried out as it flows through the aquifer before it is pumped.
The lower the degree of treatment provided by the natural environment, the lower the proportion of natural water mixed with recycled water, and the higher the degree of treatment required for returning water, requiring a higher level of treatment to remove organic matter and pathogenic bacteria. Figure 5 shows the traditional treatment processes used by the Upper Occoquan Wastewater Management Department (UOSA). The UOSA plant collects raw sewage, including domestic sewage, commercial wastewater, and industrial wastewater discharged from the standard, from the service area. After treatment, the effluent is close to the drinking water quality standard and is discharged to the Occoquan Reservoir, which is the main drinking water source in northeastern Virginia. . The treatment process is traditional primary treatment and secondary treatment with nitrification, lime recarbonation treatment, filtration, adsorption of granular activated carbon, and finally disinfection treatment. Lithium clarification treatment at pH>11 serves to disinfect, remove high molecular organic matter, and block heavy metals. Granular activated carbon further removes dissolved organic matter, especially non-biodegradable organic matter that cannot be removed from the biological treatment of the previous process. Chlorination is the last step.
UOSA facilities have been in operation since 1978 and have demonstrated the ability to meet routine specific emission standards and protect public health. Its current capacity is 130,000 m3/d and is expanding to 210,000 m3/d. Water reuse in the Occoquan reservoir (UOSA plant effluent) accounted for less than 10% in the rainy season and over 90% in the dry season.
One of the emerging processes is secondary processing followed by microfiltration (MF), reverse osmosis (RO) and ultraviolet disinfection (UV) (AWWA, 1996). MF can remove particulate matter in a wide range, and it is necessary to put it in front of the RO treatment process. RO extensively removes organic and inorganic materials, and the two-stage sequential membrane treatment process also provides extensive disinfection. UV disinfection is another protective barrier against the invasion of pathogenic bacteria. As mentioned above, the MF followed by the RO process has also become an emerging technology for the production of various industrial waters. Another emerging processing technology is the membrane bioreactor, which replaces the traditional secondary sedimentation tank with a biofilm placed outside the reactor or immersed inside the reactor (Gunder, 2001; Stephenson, et al., 2000). The necessary biomass is left on the membrane to treat the sewage and also to remove particulate matter, so the particulate matter content of the effluent is very low. The produced effluent can be used for many reuse purposes, or can be used for non-direct drinking water reuse by activated carbon adsorption, RO and UV disinfection.
8. Summary and conclusion
In summary, the relationship between the water quality required for different reuse methods and the corresponding processing technology has been listed, as shown in Table 1. The treatment of a wide range of water quality and its corresponding production reuse depends on different reuse targets. Basic secondary treatment, or a lower level of treatment, is sufficient for agricultural recycling. This kind of agricultural recycling generally does not allow the public to come into contact with water, and the crops that are watered are crops that are not directly consumed by humans and/or that require further processing. Since the biological pond treatment process can stabilize the biodegradable organic matter in the entering sewage and has the function of disinfection, the biological pond can also combine the seasonal water storage with the treatment system, so that the water supply amount and the agricultural irrigation water demand are more consistent, thus Often used. In all cases, the quality of toxic substances from industrial wastewater should be controlled, industrial wastewater should not be discharged into the sewage collection system to be reused, and (or) the factory should be required to remove toxic substances from the wastewater before being discharged into the water collection system. .
Secondary effluent is suitable for many uses after moderate disinfection (100-1000 fecal coliforms/100mL), including limited contact reuse (eg some forms of irrigation and non-contact landscape water), some ways Environmental restoration and many industrial reuse. Some environmental restorations require the removal of nutrients and enhanced disinfection. When used as industrial cooling water, it is required to remove hardness, ammonia and dissolved solids. When used as a feed water supplement, there are a number of treatments that are required, depending on the degree of dilution of the return water and existing feed water source, and whether it has been treated, such as when flowing through the soil. Membrane treatment systems are emerging in the field of reclaimed water applications. One of the emerging membrane treatment processes consists of secondary effluent as raw water, consisting of microfiltration (pretreatment), reverse osmosis, and sometimes UV disinfection. Microfiltration membranes and ultrafiltration membranes can also be incorporated into biological treatment processes to replace conventional secondary sedimentation tanks.
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