Engineering & Quality Control Specialist Report（Ⅱ）

Glen T. Daigger

5.4.1.2 Advanced Integrated Pond Systems

Advanced integrated pond systems (AIPS) represent another approach to lagoon systems that couples an initial anaerobic lagoon followed by facultative or aerated lagoons. The anaerobic lagoon is designed to provide optimum anaerobic pretreatment while also minimizing odor emissions. A variety of options are available to treat the anaerobic lagoon effluent, including high rate oxidation ponds (HROP) and facultative/aerated ponds.

One of the essential features of AIPS treatment is the initial anaerobic lagoon, illustrated in Figure 5-2. The lagoon is relatively deep and consists of an anaerobic pit in the lower reaches of the lagoon and an overlying aerobic section. Influent flow enters the bottom of the anaerobic pit and flows up through an accumulated sludge blanket. Suspended solids in the influent wastewater settle in the anaerobic pit where they undergo anaerobic stabilization. The anaerobic bacteria present in this sludge blanket also metabolize some of the colloidal and dissolved organic matter contained in the influent wastewater. The result is significant removal of both TSS and BOD5. The anaerobic pit effluent then enters the overlying aerobic zone where aerobic biodegradation of reduced compounds formed in the anaerobic sludge blanket and of a portion of the influent organic matter not removed in the anaerobic pit occurs. Oxygen is supplied to this zone by algae, the recirculation of oxygenated water from a downstream aerobic lagoon, and/or mechanical aerators. The presence of the aerobic layer over the anaerobic pits helps to control odor emissions.

figure 5-2 Anaerobic Lagoon for Advanced Integrated Pond (AIPS) System.

The effluent from the anaerobic lagoon then passes into the downstream lagoon system where several options are available, including HROP, facultative lagoons, and aerated lagoons. These lagoons provide removal of biodegradable organic matter present in the anaerobic lagoon effluent, disinfection, and a certain degree of nutrient removal. Effluent quality will depend significantly on the control of algae growth and separation in this portion of the treatment system. The design of this portion of the system can be adjusted, depending on process objectives (Oswald, et al., 1994; Oswald, 1991).

Conversion of a conventional lagoon to an AIPS can significantly affect the performance of the system. Anaerobic lagoons can remove BOD5 and TSS efficiently and with virtually no energy input. The removal of heavy metals and certain chlorinated organics also may be good. Sulfate will be reduced to sulfide in the anaerobic pit, and the sulfide will precipitate heavy metals contained in the influent wastewater. The anaerobic conditions in the anaerobic pit also can result in dechlorination of chlorinated organic compounds, thereby enhancing their biodegradation in the downstream aerobic portions of the treatment system. Suspended solids removed from the influent wastewater are stabilized anaerobically and stored in the anaerobic pit. Stabilized solids can be stored for several years before they must be removed.

The significant removal of organic matter in the anaerobic lagoon will reduce the loading on the downstream portion of the treatment system. Consequently, plant treatment capacity usually will be increased significantly. As with conventional lagoon systems, effluent quality will be determined largely by the concentration of algae in the effluent.

5.4.1.3 PETRO

The term PETRO stands for Pond Enhanced Treatment and Operation and represents a coupling of a lagoon wastewater treatment system with a trickling filter. The trickling filter is used to remove algae from the effluent of the facultative pond and upgrade effluent quality.

The lagoon treatment system used in the PETRO process consists of an anaerobic lagoon similar in configuration to that used in the AIPS, followed by a facultative lagoon. Effluent from the facultative pond is recycled to the anaerobic pond to maintain an overlying layer of oxygenated water to control odors. This process is very energy efficient. Significant removal of suspended solids and biodegradable organic matter occurs in the anaerobic pond where no energy input is required. Significant removal of biodegradable organic matter also occurs in the facultative pond, along with the removal of nutrients and the inactivation of pathogens, without the input of external energy. However, the effluent can contain relatively high concentrations of algae.

The algae are removed from the facultative lagoon effluent in the trickling filter. Facultative lagoon effluents have been treated in trickling filters. While significant nitrification has been observed in such systems, poor algae removal was observed. The developers of the PETRO process hypothesized that this was caused by the absence of readily biodegradable organic matter to allow the growth of heterotrophic bacteria. To provide this, they decided to divert a portion of the anaerobic lagoon effluent around the facultative lagoon and directly into the trickling filter. As hypothesized, this resulted in the growth of a heterotrophic biomass within the trickling filter that was able to remove significant quantities of algae from the facultative lagoon effluent. When this combined heterotroph and algal biomass subsequently sloughed off the trickling filter, it was found to settle readily in the downstream clarifier. Nitrification also was observed in the trickling filter. Figure 5-3 provides a schematic of the PETRO process (Meiring, et al., 1990; Meiring and Hoffman, 1994).

figure 5-3 PETRO Process.

The PETRO process offers the potential to significantly upgrade the effluent from an existing facultative lagoon process while also minimizing process energy requirements. A lagoon treatment system consisting of an anaerobic lagoon followed by a facultative lagoon requires little energy input. The only energy input required is to pump facultative lagoon effluent to recycle it to the anaerobic lagoon to maintain a surface layer of oxygenated water. Trickling filters also are energy efficient processes. The primary energy requirement is to pump influent wastewater and recycle flow to the trickling filter. The principal benefit of the trickling filter is to provide a mechanism to remove algae from the facultative lagoon effluent. This will result in a significant improvement in effluent quality. Nitrification also can occur in the trickling filter and reduce ammonia-nitrogen concentrations in the process effluent. Some degree of nitrogen removal will occur in the anaerobic lagoon as facultative lagoon effluent, which will be at least partially nitrified, is recirculated to the anaerobic lagoon. As with AIPS, significant removal of heavy metals will occur in the anaerobic pits in the anaerobic lagoon, and chlorinated organics will be dechlorinated and facilitate biodegradation in the downstream facultative lagoon and trickling filter.

The capacity of an existing lagoon wastewater treatment plant generally can be increased substantially by implementing the PETRO process. Substantial quantities of biodegradable organic matter are removed in the anaerobic lagoon, thereby allowing increased flows to be treated there. Moreover, not all of the influent flow passes through the facultative lagoon. Finally, complete treatment is not required in the facultative lagoon because further treatment will occur in the downstream trickling filter.

5.4.2 Advanced Primary Treatment

Advanced primary treatment involves the use of a metal salt to destabilize colloidal particles in a wastewater and an organic polymer to build destabilized colloids into large, settleable flocs that can be removed in conventional primary clarifiers (Daigger and Buttz, 1998). The result is a process that can remove most of the settleable and colloidal particles from raw wastewater. Truly dissolved organic matter is not generally removed. For many domestic wastewaters between 80 and 90 % of the influent TSS can be removed, while between 50 and 60 % of the influent BOD5 can be removed. Phosphorus is also removed effectively due to use of metal salts (aluminum and iron).

While advanced primary treatment offers the potential for enhanced BOD5, TSS, and phosphorus removal, it does not provide for effective removed of nitrogen or dissolved BOD5. Consequently, it does not produce an effluent that effectively protects water quality. Advanced primary treatment can be an element of an overall wastewater treatment process train that includes a biological nitrogen removal process downstream of the advanced primary treatment system. In this case, advanced primary treatment is used to reduce the size of the downstream biological nitrogen removal facility. Advanced primary treatment can generally be used as the principal wastewater treatment process in only one instanced - prior to ocean disposal of an effluent through a deep ocean outfall. Since advanced primary treatment effluent is not appropriate for discharge to fresh water or for reuse, it must be disposed of. Moreover, since nitrogen has not been removed, it cannot be discharged into shallow coastal waters since it will result in eutrophication of those waters. Instead, an outfall sufficiently long to transport the advance primary treated effluent beyond coastal waters and into deep ocean waters is required.

Advanced primary treatment has been considered recently as a wastewater treatment alternative for several projects in China. From the above it is clear that advanced primary treatment, alone, will have limited applications - only for those situations where deep ocean outfalls are available. It can, however, be a component of an overall process train that also includes downstream biological nitrogen removal. Advanced primary treatment can also be used in the phased implementation of wastewater treatment at a particular location when sufficient funds are not available to initially construct the entire wastewater treatment plant. The primary clarifiers can be constructed and operated in an advanced primary mode in one phase of overall plant construction. Later, biological treatment facilities can be added downstream of the primary treatment system. The decision to continue or to stop adding chemicals to the primary clarifiers when the biological treatment facilities are subsequently constructed would be based on economics.

5.4.3 Biological Nutrient Removal

Biological nutrient removal (BNR) is a component of modern wastewater treatment facilities and will certainly be a component of wastewater treatment facilities to be constructed in China. A variety of BNR removal processes capable of removing nitrogen, phosphorus, or both nitrogen and phosphorus are available. Both suspended growth and fixed film processes are available (Grady, Daigger, and Lim, 1998). Suspended growth processes are well developed. A variety of process options are available, each with its own advantages and disadvantages. There appears to be little reason to standardize on a single suspended growth BNR process. Rather, the alternative processes would be evaluated, as described in section 4.3, and the most appropriate process selected. Advances continue to be made, including development of the step feed biological nitrogen and phosphorus removal process (Nolasco, et al., 1998). Figure 5-4 provides a schematic of the step feed biological nitrogen and phosphorus removal process, referred to as the step bio-P process. Benefits of the step bio-P process include reduced bioreactor volume and a reduction in the size and complexity of mixed liquor recycle pumping facilities. Another interesting development is the explicit use of simultaneous biological nutrient removal in BNR processes (Littleton and Daigger, 1999).

Figure 5-4. Step Bio-P Biological Nitrogen and Phosphorus Removal Process.

Suspended growth BNR facilities can be implemented either with or without upstream primary treatment. This decision is driven largely by economics and sludge handling process considerations. The provision of primary treatment also results in the presence of primary sludge that can be fermented to increase the efficiency of biological phosphorus removal processes. If primary treatment is not provided, alternative approaches to fermentation may be required to ferment incoming wastewater solids to support efficient biological phosphorus removal.

Sequencing batch reactors (SBRs) can also be used for biological nitrogen and phosphorus removal. SBRs are cost-effective for small wastewater treatment facilities, but their cost-effectiveness diminishes for larger facilities. They are viable alternatives, but they may not offer unique alternatives for the setting in China.

Fixed film biological nitrogen removal processes have also been developed and are available. Their principal advantage is that they require less space than a comparable suspended growth BNR system. However, the fixed film systems are generally more expensive than suspended growth processes. Moreover, the fixed film processes are more likely to require an external carbon source, such as methanol, to achieve effective biological nitrogen removal. Achieving biological phosphorus removal is also more difficult with fixed film processes as it is difficult to achieve the anaerobic/aerobic cycling required for biological phosphorus removal using fixed film processes. Specific applications may exist where fixed film processes may offer significant advantages. But, it appears that this will not be the general case in China.

5.4.4 Disinfection

As discussed above in section 5.2, concern is increasing over the control of pathogens through improved wastewater treatment. Wastewater disinfection is accomplished at four levels:

- One level is through the wastewater treatment process itself. Wastewater treatment processes remove pathogens by both removing suspended solids, which have pathogens associated with them, and by allowing pathogens to die off.
- Another level is by disinfection processes, including chlorine and ultraviolet disinfection.
- The third level is through the control of particulate matter in the effluent. The removal of particulates results in the direct removal of pathogens. It also makes disinfection processes, including chlorine and ultraviolet, more effective.
- The fourth level is the location where the treated effluent is introduced into the environment. For example, the use of a properly designed diffuser that introduces the treated effluent into a sizeable receiving stream allows for more natural die-off of pathogens before they can come into contact with humans than if raw sewage is allowed to flow into a small, local stream.

The future certainly will include the extension of additional wastewater service throughout China, which will accomplish the first and fourth levels of disinfection. Formal disinfection processes, such as chlorine and ultraviolet, will be incorporated into the wastewater treatment plants built in support of the extension of urban wastewater service.

Increasing reuse of treated effluent should be associated with increased control of effluent particulates. Granular media filters are traditional unit processes used for this purpose. Membranes are an emerging technology, as discussed in section 5.4.6. Membranes will have only specific options, however, due to their relatively high cost.

Another option is use of natural features for enhanced particulate control, such as rapid infiltration basins. In this option, sandy soils are located where treated effluent can be applied and allowed to infiltrate into the ground. Depending on ground water location and flow, the treated effluent can be allowed to either flow into the ground water, or it can be recovered by shallow wells. Passage of the effluent removes particulates, pathogens, and other pollutants. The effluent can be used to recharge groundwater, or it can be recovered directly for use. Such a system provides effective treatment and pathogen control and can result in a water that can be effectively reused.

5.4.5 Anaerobic Treatment

An interesting development is the potential to apply anaerobic treatment to municipal wastewater (van Haandel and Lettinga, 1994). If extraneous water can be excluded, thereby reducing wastewater volumes, increasing wastewater strength, and raising wastewater temperatures, anaerobic treatment may be a cost-effective option. In fact, anaerobic ponds have been found to be effective for the treatment of municipal wastewater, as described in connection with the PETRO process discussed in section 5.4.1.3 above. As discussed there, anaerobic treatment offers significant potential to reduce energy requirements and decrease sludge production from municipal treatment systems. The potential also exists to obtain additional toxics removal, including heavy metals, chlorinated organics, and other recalcitrant organics. China should follow developments in this area, and potentially conduct pilot- and demonstration-scale studies.

5.4.6 Membranes

Interest is developing around the world in the use of membranes for water and wastewater treatment. For wastewater treatment systems this increased interest focuses on the use of membrane bioreactors, where the conventional secondary clarifier is replaced with a microfiltration or ultrafiltration membrane process. Filtration of mixed liquor by the membrane replaces gravity sedimentation as the mechanism to retain and concentrate biomass in the process. Because higher mixed liquor suspended solids (MLSS) concentrations can be achieved using membranes for liquid/solids separation, smaller bioreactors can be used. Conversely, long solids residence times (SRTs) can be maintained because of the high MLSS concentrations that can be achieved, thereby allowing extensive biological treatment to occur. Moreover, because membrane processes provide for more effective removal of particulate matter, a very high quality effluent can be produced. The effluent from membrane bioreactor processes is well prepared for a variety of reuse options.

In spite of their advantages, membrane bioreactors systems are still expensive (due to the cost of the membranes) and are not yet cost competitive with conventional suspended growth biological processes for typical applications. However, membrane bioreactors can be cost-effective for applications requiring high quality effluents, especially for reuse applications. Membrane performance also continue to improve and membrane costs continue to decrease, increasing the range of applications where membrane bioreactors are cost-effective. This is a technology that China should be following carefully.

5.5 Solids Processing Technologies

The processing and disposal of wastewater sludge is a necessary component of an effective wastewater treatment facility. This section discusses some technologies particularly applicable to China. Topics to be addressed include:

- Solids Disposal Options.
- Aerobic Digestion.
- Advanced Digestion Options.
- Alkaline Treatment.
- Solar Drying.
- Sludge Thickening and Dewatering.

5.5.1 Solids Disposal Options

Any discussion of wastewater treatment plant solids processing technologies must begin with a discussion of sludge reuse and disposal options. This is because the purpose of solids processing is to prepare the solids for ultimate reuse or disposal. While a variety of specific options exist, they can be grouped into agricultural reuse, landfilling, and disposal to the air through incineration. Ocean disposal is a historical practice that has largely been eliminated due to the adverse environmental impacts.

A variety of sludge streams can be landfilled, including raw, dewatered sludge; stabilized, dewatered sludge; and incinerator ash. In general, landfilling represents the waste of potentially valuable resources (the nutrients and organic matter in the sludge). Landfilling also causes adverse environmental impacts such as leachate; landfill gases; and consumption of land that cannot, subsequently, be used for other purposes. In general, landfilling of wastewater treatment sludge should be discouraged. Incineration is an expensive, mechanically intensive option. While its environmental impacts can be relatively modest if appropriate air pollution controls are used, the expense and complexity of this option make it undesirable for the situation in China. Thus, reuse of wastewater sludge, particularly for agriculture, appears to be the best option for China.

Reuse of the nutrients and organic matter in wastewater sludge through land application is a historical practice. It is also environmentally and economically beneficial. The nutrients and organic matter in the sludge are used to provide nutrients for agriculture and to increase the water holding capacity of soils. Two principal environmental concerns are the potential presence of toxic materials, such a heavy metals, and of pathogens (U.S. EPA, 1989). Toxics can be effectively controlled by preventing their initial discharge to the wastewater collection system through the implementation of an effective industrial pretreatment program. Pathogens can be controlled by the use of appropriate treatment processes, as described below. Another consideration is stabilization of the sludge, i.e. reduction in the fraction of biodegradable organic matter. Stabilization of the sludge reduces aesthetic concerns associated with sludge reuse, and it reduces the attractiveness of sludge to vectors such as flies and rodents.

Thus, the principal objectives of sludge treatment for most applications in China should be to prepare it for economical reuse. Toxics are controlled by an effective industrial pretreatment program. In-plant sludge treatment processes are used to reduce pathogens, stabilize biodegradable organic matter, and remove water (thicken and dewater) to reduce transportation costs.

5.5.2 Aerobic Digestion.

Aerobic digestion is a historical process used to stabilize waste sludges from wastewater treatment plants. Traditionally it had been designed as a single process reactor where waste sludge is added to it and digested sludge is removed from it. Operation of the aeration device may periodically be turned off to allow solids to settle and removal of a relatively clear supernatant. This allows the solids residence time to be increased above the hydraulic residence time in the digester. Sludge is stabilized as biodegradable organic matter is oxidized, and pathogens decline as a result of natural die-off.

While organic matter stabilization is achieved with aerobic digesters operated in this historical fashion, pathogen destruction is relatively poor. Moreover, pH can decline precipitously as a result of alkalinity destruction by nitrification. The temperature in the aerobic digester can also decline during cold weather operation due to the relatively long hydraulic residence time.

More recently, options have been developed to improve the performance of aerobic digestion (Bailey and Daigger, 1998; Daigger, et al., 1997). Improvement options include:

- Pre-Thickening. The sludge is thickened prior to addition to the digester. Feed sludge concentrations in the 3 to 5 % solids range can be used.
- Staged Operation. Digesters are operated either in a tanks-in-series mode, or in a semi-batch mode.
- Aerobic/Anoxic Operation. The oxygen transfer system is cycled on and off to allow nitrification to occur during the aerated period and denitrification to occur during the non-aerated period.

Pre-thickening reduces the sludge volume, thereby reducing the digester volume required to achieve a specified SRT. Reducing the amount of water added to the digester also allows the heat of combustion of biodegradable solids to be used to heat the digester, thereby avoiding low wintertime temperatures. Experience suggests that an insulated cover over the digester may be required in cold climates to achieve effective temperature increases. Note, auto-heating of the digester above about 35 oC must be avoided during warmer summertime conditions to avoid foaming and other operating problems. This can be accomplished by adding some un-thickened sludge into the digester and thickening the digesting sludge to maintain the desired SRT.

Staged operation creates plug flow conditions that result in improved digestion (plug flow reactors are inherently more efficient than completely mixed reactors) and improved destruction of pathogens. Pathogens are destroyed more effectively because short-circuiting of pathogens through the digester is significantly reduced when plug flow conditions are created. Figure 5-5 illustrates two methods for achieving plug flow conditions.

figure 5-5 Staged Operation of Aerobic Digesters by Series Operation or Semi-Batch Operational Mode.

Cycling the oxygen transfer system on and off allows denitrification to occur. One result is reduced alkalinity consumption due to the alkalinity recovered by denitrification. It is theoretically and practically possible to maintain near neutral pH values through this technique. Energy requirements are also reduced as the nitrate-nitrogen produced as a result of nitrification is used to stabilize biodegradable organic matter. Nitrogen is also removed, which allows more sludge to be applied per unit area of land before agricultural nutrient requirements are exceeded. Table 5-1 summarizes the impacts of the new aerobic digestion features on digester performance.

Autothermal Thermophilic Aerobic Digestion (ATAD) is another option (U.S. EPA, 1990). Experience suggests, however, that ATAD systems are relatively difficult to operate and prone to odor problems. Moreover, the digested sludge is relatively difficult to thicken and dewater. Thus, while it is an option, its increased complexity and operational difficulties make it a less attractive option than once thought.

Table 5-1 Impacts of Aerobic Digester Improvement Techniques on Digester Performance. Technique Impacts Pre-Thickening Reduced Sludge Flow Rate Reduces Volume to Achieve Target SRT.Increased Temperature. Staged Operation Improved Sludge Stabilization.
Improved Pathogen Destruction. Aerobic/Anoxic Operation Reduced Alkalinity Consumption.
Reduced Energy Requirements.
Nitrogen Removal.

5.5.3 Advanced Digestion Options.

The term advanced digestion refers to a variety of improvements to the traditional mesophilic anaerobic digestion process that include:

- Two-Stage, Acid/Gas Operation.
- Mesophilic versus Thermophilic Operation.
- Pre-Pasteurization.
- Aerobic, Autothermal Thermophilic Pre-Treatment.

Two-stage operation involves separation of the acid forming anaerobic digestion step (the acid phase) from the methane formation step (the gas phase) by the operation of digesters in series. The first digester will be operated at relatively low SRTs (and HRTs) so that particulate matter will be hydrolyzed and volatile acids will be formed but only limited methane formation will occur. The SRT (and HRT) in the second stage will be sufficiently long to allow methanogens to grow, resulting in conversion of the volatile acids formed in the acid phase, and those also formed in the gas phase, to methane gas. Potential advantage of this process include:

- Increased destruction of particulate matter due to increased hydrolysis in the more acid environment created in the acid phase digester. The result is greater destruction of volatile solids, resulting in less digested sludge to be disposed of.
- Increased pathogen destruction, both because the digester is staged (more plug flow conditions) and because the acidic conditions created in the acid phase digester cause pathogens to be inactivated more quickly.
- Reduced foaming in the gas phase digester because potential foam causing materials (such as the nuisance microorganisms Nocardia) are hydrolyzed in the acid phase digester.

Thermophilic operation involves maintenance of temperatures in the digester of about 55 oC, in comparison to a temperature of 35 oC for mesophilic operation. Potential advantages of thermophilic operation include:

- Increased pathogen inactivation by the higher temperature maintained.
- An increased rate of digestion, thereby allowing use of lower SRT values.

It is generally observed, however, that sludges digested under thermophilic conditions are not as well stabilized as those digested under mesophilic conditions. They are more odorous. Moreover, they often do not dewater as well in mechanical dewatering processes.

Pre-pasteurization involves heating sludge to 70 oC and holding it for 30 minutes prior to adding it to the anaerobic digester. These are essentially the same conditions used to pasteurize milk, and it is done to fully inactivate pathogens. The result is an essentially pathogen-free sludge.

Aerobic, autothermal, thermophilic pretreatment involves use of a high-rate ATAD unit to pre-heat the sludge prior to conventional mesophilic anaerobic digestion. Thus, mechanical heating equipment is not needed. However, the ATAD pre-treatment process must be operated to achieve a temperature that optimizes the downstream mesophilic anaerobic digester.

Of these options, the most current interest is focused on:

- Thermophilic Acid Phase/Mesophilic Gas Phase (TA/MG) Anaerobic Digestion.
- Mesophilic Acid Phase/Mesophilic Gas Phase (MA/MG) Anaerobic Digestion.

TA/MG offers enhanced destruction of volatile solids, excellent pathogen destruction, and the production of a very stable product (since the final digestion stage is mesophilic). The extra heat added to elevate the temperature of the thermophilic acid phase digester is recovered to heat incoming sludge. Consequently, the energy balance is actually quite reasonable. MA/MG offers most of the advantages of TA/MG operation, along with a system that is mechanically somewhat simpler. The principal unanswered question is how much additional pathogen destruction is achieved by thermophilic operation of the acid phase digester, and whether it is worth the additional complexity and modest increase in operating costs. This question is yet to be answered.

Pre-pasteurization is being considered as a method to upgrade the pathogen destruction achieved with conventional digesters. In fact, pre-pasteurization could be coupled with a two-stage acid/gas phase digester to achieve control of pathogens and enhanced digestion.

Work is on-going in the United States and Europe with advanced digestion options. China should monitor the experiences in these two areas and potentially incorporate some of these features in future anaerobic digester designs.

5.5.4 Alkaline Treatment.

Alkaline treatment involves the addition of various alkaline agents to unstabilized sludge to reduce pathogens and reduce biological activity. Alkaline agents used include lime, cement kiln dust, and incinerator fly ash. Significant amounts of cement kiln dust or incinerator ash must be added to achieve the pH elevation required to control pathogens in the raw sludge. However, when applied to acid soils both the nutrients in the sludge and the alkaline material are beneficial to agriculture. When sufficient quantities of suitable alkaline materials are available as industrial by-products, and local soils are acid so that application of alkaline material will enhance their productivity, alkaline treatment of sludge may be a viable and cost-effective option.

5.5.5 Solar Drying

Solar drying of digested, dewatered sludge is being used in a number of localities as a method to reduce pathogens. For example, aerobic digestion can be used to biodegrade the majority of the biodegradable organics, resulting in a sludge that can be stock-piled without causing odor problems. The aerobically digested sludge is dewatered, and then stock-piled for a period of several months to allow pathogens to die off. Substantial drying can also occur, resulting is significant volume reduction. This has proven to be an excellent method for improving the hygienic quality of sludge for subsequent land application.

5.5.6 Sludge Thickening and Dewatering.

Sludge thickening and dewatering options have not changed much over the past several years. The conventional technologies are listed in Table 5-2.

Table 5-2 Sludge Thickening and Dewatering Options. Thickening Dewatering

Gravity.
Dissolved Air Flotation (DAF).
Gravity Belt Thickener.
Centrifuge

Belt Filter Press.
Centrifuge.
Plate and Frame Press.

No new sludge thickening and dewatering technologies have been introduced in the past several years. However, the existing technologies continue to evolve, and improvements particularly in the belt thickening and dewatering and the centrifuge technologies occur on a regular basis. These changes should be followed on a routine basis to ensure that the most cost-effective technology is being used.

5.6 References

1 Bailey, E. and G. T. Daigger, "Improving Aerobic Digestion to Meet Class B Requirements by Pre-Thickening, Staged Operation and Aerobic/Anoxic Operation: Four Full-Scale Demonstrations," Proceedings, Water Environment Federation 71st Annual Conference & Exposition, Volume 2, Residuals & Biosolids Management and Collection Systems, 303-314, 1998.

2 Daigger, G. T. and J. A. Buttz, Upgrading Wastewater Treatment Plants, Second Edition, Technomic Publishing Co., Lancaster, PA, 1998.

3 Daigger, G. T., S. P. Graef, and S. Eike, "Conversion of a Conventional Aerobic Digester to the Aerobic/Anoxic Digestion Process," Proceedings, Water Environment Federation 70th Annual Conference & Exposition, Volume 2, Residuals & Biosolids Management, Collection Systems, 287-297 1997.

4 Grady, C. P. L., Jr., G. T. Daigger, and H. C. Lim, Biological Wastewater Treatment, Second Edition, Marcel Dekker, NY, 1998.

5 Littleton, H. X. and G. T. Daigger, "Mechanism for Simultaneous Nitrification/Denitrification and Biological Phosphorus Removal in Orbal Oxidation Ditches and Their Full-Scale Application," Book of Technical Papers with Conference Programme, Water Industries Conference-Hong Kong 1999, 21st Century Perspective of Water Supply & Sewerage, Hong Kong, 19-22 January, 1999, 478-487.

6 Mara, D. Low-Cost Urban Sanitation, John Wiley & Sons, NY, 1996.

7 Meiring, P. G. J. and J. A. Van Huyssteen, "Upgrading of Wastewater Treatment Plants: Oxidation Ponds in Series with Biological Filters," Water Science and Technology, 22(7/8), 291-292, 1990.

8 Meiring, P. G. J. and J. R. Hoffmann, "Anaerobic Pond Reactor In-Line With Biological Removal of Algae," International Association on Water Quality Seventh International Symposium on Anaerobic Digestion, Cape Town, SA, 1994.

9 Nolasco, D. A., G. T. Daigger, D. R. Stafford, D. M. Kaupp, and J. P. Stephenson, "The Use of Mathematical Modeling and Pilot Plant Testing to Develop a New Biological Phosphorus and Nitrogen Removal Process," Water Environment Research, 70, 1205-1215, 1998.

10 Oswald, W. J., "Introduction to Advanced Integrated Wastewater Ponding Systems," Water Science and Technology, 24(5),1-7, 1991.

11 Oswald, W. J., F. B. Green, and T. J. Lundquist, "Performance of Methane Fermentation Pits in Advanced Integrated Wastewater Pond Systems," Water Science and Technology, 12(30), 287-296, 1994.

12 Rich, L. G., High Performance Aerated Lagoon Systems, American Academy of Environmental Engineers, Annapolis, MD, 1999.

13 Rich, L. G., "Design Approach to Dual-Power Aerated Lagoons," Jour. of the Environmental Engineering Division, ASCE, 108, 532-548, 1982.

14 Rich, L. G., "Influence of multicellular configurations on algal growth in aerated lagoons," Water Research, 16, 929-931, 1982.

15 Rich, L. G. and B. W. Connor, 1982. "Benthal Stabilization of Waste Activated Sludge," Water Research, 16, 1419-1423, 1982.

16 U.S. Environmental Protection Agency, Autothermal Thermophilic Aerobic Digestion of Municipal Wastewater Sludge, EPA/625/10-90/007, Washington, DC, 1990.

17 U.S. Environmental Protection Agency, Control of Pathogens in Municipal Wastewater Sludge, EPA/625/10-89/006, Center for Environmental Research Information, Cincinnati, OH, 1989.

18 Van Haandel, A. C. and G. Lettinga, Anaerobic Sewage Treatment: A Practical Guide for Regions with Hot Climates, John Wiley & Sons, NY, 1994.

6. Strategy for Private Sector or Other Sector Involvement

6.1 Introduction

This section addresses TOR element number five for the Engineering & Quality Control Strategy Specialist. This element is to develop a strategy of private sector or other sector involvement in water and wastewater services. The topics to be addressed include:

- When is private or other section involvement beneficial?
- Under what circumstances will private or other sectors be interested in involvement with China?
- How can private and other sector involvement be encouraged, when it is beneficial?

6.2 Benefits of Private and Other Sector Involvement

China can benefit from the involvement of the private sector, and other sectors in water and wastewater service, when these other sectors bring an element that is lacking, either from within China or from within the agency providing the water and wastewater service. Examples of these elements include:

- Technology.
- Management Expertise.
- Capacity to Produce Necessary Service Components.
- Short-Term Financing.

Technology involves a wide array of elements, from knowledge of water and wastewater conveyance and treatment planning and design options, to knowledge about effective management and construction techniques. Equipment technology is particularly important to China presently, as significant numbers of new facilities will be constructed. If the equipment necessary to construct those facilities can be manufactured in China, the cost will be significantly reduced and modern water and wastewater service can be extended to the Chinese population at a faster pace. The result will be a faster improvement in public health, preservation of available water resources, and increased protection of the environment.

Management expertise includes both the overall management of water and wastewater programs on a countrywide scale, and the ability to efficiently and effectively operate local water and wastewater utilities. Without effective and efficient management structures, new facilities will not be constructed fast enough, and those which are constructed will not be efficiently operated and maintained.

Production capacity refers to the need to provide a sufficient quantity of all necessary system components to allow water and wastewater services to be extended at a sufficient rate to meet the demand. For example, China, and its water and wastewater service sector, may possess the capability to produce all needed system components, but their capacity may not be sufficient to allow water and wastewater services to be extended at the necessary pace. In this case, private and other sectors can assist by providing the difference between the demand and the resident capacity. This is a traditional role of the private sector in all elements of public infrastructure, including water and wastewater service.

Short-term financing is necessary to allow capital facilities to be implemented in a timely fashion. In some instances, this short-term financing can be provided by the private sector, by a non-governmental agency, or by a governmental agency from outside of China. However, it is important to understand that, on a long-term basis, financing of water and wastewater services must come from the system users.

6.3 Interests for Private and Other Sector Involvement in China

The interests of the private sector are quite simple. The private sector is interested in establishing a viable and profitable business enterprise. This business enterprise will ideally be long-term in nature. If the business opportunity is short-term, then the profit from it must be relatively high to justify the costs necessary to acquire the business and to mobilize the resources required to provide the associated service and/or product. A long-term opportunity, on the other hand, does not demand the same level of profitability as a short-term one, if there is a level of assurance that the long-term opportunity will continue to be available as long as the level of service is satisfactory. From a private sector perspective, an on-going business opportunity is one that can be provided profitably and that will continue as long as an acceptable level of service is provided to the customer.

An issue of concern to the private sector is risk. The private sector recognizes that risk is inherent in any business venture, and it develops and implements procedures to mitigate these risks. However, the private sector is generally unwilling to accept the responsibility and/or consequences of risks that are beyond their control. In short, the private sector is willing to be responsible for what it can control but to be absolved from risks associated with elements beyond its control. Some simple examples may be illustrative here. When the private sector is responsible for the operation of a wastewater treatment plant, the private sector is willing to be responsible for the performance of the plant as long as the loadings on the plant are within its design values. But, if wastewater flows and/or pollutant loadings exceeded design values, the private sector would not be willing to accept the responsibility for meeting performance requirements. This is because the actions required to achieve acceptable performance are beyond the control of the private sector.

A risk of special concern to the private sector is the payment risk. If payment cannot be reasonably expected for the services provided, the private sector must increase its price or seek other methods to mitigate the risk. Thus, one of the best approaches that China and Chinese agencies can use to reduce the cost of services and products that will be provided by the private sector is to provide a high degree of assurance that payment will be made for the services and products they provide.

Many business deals seek to transfer the responsibility for elements beyond the control of the private sector contractor to that contractor. The result is often unsatisfactory for both the private sector contractor, and for the contracting agency. This is because it is a less capable private sector business that will accept such unfair contract terms, resulting in the selection of a private sector contractor that may not be capable of providing the required service and/or products with the necessary quality.

Other sectors, such as non-governmental agencies and governments outside of China, will have a variety of other interests. Many times these interests are political in nature, and they may either be in alignment or in conflict with Chinese political interests. In other cases they interest will be economic in nature - to develop a position that will secure a long-term business opportunity. Thus, the interests of any such agency or government must be carefully analyzed to determine that they are in alignment with Chinese interests.

6.4 A Strategy for Private and Other Sector Involvement in Chinese Water and Wastewater Services

The recommended strategy for encouraging and obtaining private and other sector involvement in Chinese water and wastewater services includes the following elements:

- Identify capacity limiting elements in the current Chinese water and wastewater service sector.
- Identify private and other sectors that can assist with strengthening these overall capacity limiting elements.
- For private sector involvement opportunities, examine alternative private sector involvement models. When analyzing these alternative models, remember that that the private sector values long-term business opportunities with clear success measures and where a reasonable profit can obtained. The private sector is willing to accept the responsibility for risks that it can control, but not for risks outside of its control. So, the model should carefully evaluate what the private sector is being made responsible for , what it is not being made responsible for, and what is allowed to control. Evaluate these models in terms of the benefit to China and the reasonableness of the model from a private sector perspective.
- Implement a variety of pilot programs to investigate the applicability of these various models to the setting in China.
- Use a similar approach for involvement from sectors other than the private sector. When evaluating the alternative models, analyze the objectives of the sector providing the service to determine whether they align with Chinese interests. If they do not align, then proceed very cautiously as the basis may not exist for a successful, long-term venture.