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Selection of Domestic Wastewater Treatment Systems in Warm Climate Regions

Regional climate and developmental status play a very important role in the conception, design and operation of biological wastewater treatment. Most of the world is situated in warm-climate and less-developed regions, but, conversely, the majority of the available technical information on wastewater treatment comes from temperate-climate and well-developed countries. This article discusses briefly some of the relevant issues related to the selection of domestic wastewater treatment processes, especially in warm-climate and less-developed regions.The implementation of wastewater treatment plants has been so far a challenge for most countries. Economical resources, political will, institutional strength and cultural background are important elements defining the trajectory of pollution control in many countries. Technological aspects are sometimes mentioned as being one of the reasons hindering further developments. However, there is awide list of technological options for the treatment of wastewater.

The influence of temperature and developmental status

Biological wastewater treatment is very much influenced by climate. Temperature plays a decisive role in some treatment processes, especially the natural-based and non-mechanised ones. Warm temperatures lead to a decrease in land requirements, enhance conversion processes, increase removal efficiencies and make the utilisation of some treatment processes feasible. For example, some treatment processes, such as anaerobic reactors, may be utilised for diluted wastewater, such as domestic sewage, in warm climate areas (Figure 2). Other processes, such as stabilisation ponds, may be applied in lower temperature regions, but occupying much larger areas and being subjected to a decrease in performance during winter (Figure 3). However, other processes, such as activated sludge and aerobic biofilm reactors, are more independent from temperature, as a result of the higher technological input and mechanisation level. Even so, nitrification is influenced by temperature, and in warm climate regions it is likely to take place, even under unfavourable conditions and low sludge ages, in activated sludge reactors (Figure 4). In these figures, the breadth of the application ranges can be seen, with installations serving from hundreds to one million inhabitants.

Another important point is that most warm climate regions are situated in developing countries. Simple, economical and sustainable solutions are strongly demanded. All technologies presented may be applied in developing regions, but of course they imply different requirements in terms of energy, equipment and operational skills. Whenever possible, simple solutions, approaches and technologies are presented and recommended.

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Fig 1. Flowsheets of Important Wastewater Treatment Systems

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Fig 2. Upflow anaerobic sludge blanket (UASB) reactor. (a) Installation for hundreds of inhabitants and (b) for one million inhabitants. In warm climate regions they may be applied for the treatment of less concentrated wastewater, such as domestic sewage (UFMG/COPASA Experimental Treatment Plant and Onça WWTP, COPASA, Brazil)

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Fig 3.

Stabilisation ponds. (a) Installation for hundreds of inhabitants and (b) for hundred thousands of population equivalents. The required areas in warm climate regions are 3 to 4 times lower than in temperate regions (UFMG/COPASA Experimental Treatment Plant and Maracanaú WWTP, CAGECE, Brazil – source Google Earth

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Fig 4. Aerobic reactors. (a)Activated sludge and (b) trickling filter. In warm climate regions, nitrification is more likely to occur than in temperate climates (Morro Alto WWTP, COPASA and Itabira WWTP, SAAE-Itabira, Brazil)

UASB reactors and post-treatment systems

Taking into consideration the intrinsic limitations associated with the anaerobic systems and the need to develop technologies that are more appropriate to the reality of developing countries, it is important to include a post-treatment stage for the effluents generated in anaerobic reactors. This stage has the purpose of polishing not only the microbiological quality of the effluents, in view of the public health risks and limitations imposed on the use of treated effluents in agriculture, but also the quality in terms of organic matter and nutrients, in view of the environmental impact caused by the discharges of the remaining loads into the receiving bodies.

In comparison with a conventional wastewater treatment plant consisting of primary sedimentation tank followed by aerobic biological treatment (activated sludge, trickling filter, submerged aerated biofilter or rotating biological contactor), with the primary and secondary sludge passing through sludge thickeners and anaerobic digesters prior to dewatering, a treatment plant consisting of a UASB reactor followed by aerobic biological treatment (with the secondary sludge directed to thickening and digestion in the UASB reactor itself and then straight to dewatering), can present the following advantages:

The primary sedimentation tanks, sludge thickeners and anaerobic digesters, as well as all their equipment, can be replaced by UASB reactors, which do not require the use of equipment. In this configuration, besides their main sewage treatment function, the UASB reactors also accomplish the sludge thickening and digestion functions, requiring no additional volume.

Power consumption for aeration in activated sludge systems preceded by UASB reactors will be substantially lower compared to conventional activated sludge systems, and especially extended aeration systems.

Thanks to the lower sludge production in anaerobic systems and to their better dewaterability, sludge volumes to be disposed of from anaerobic/aerobic systems will be much lower than those from aerobic systems alone. Values around 30% VSS destruction can be reached when secondary sludge produced in a trickling filter is returned to a UASB reactor. When the mass balance is performed, the total sludge production in a combined UASB/Trickling Filter system can be 30 to 50% lower than in a conventional trickling filter system.

The construction cost of a treatment plant with UASB reactor followed by aerobic biological treatment usually amounts to 50 to 80% of the cost of a conventional treatment plant (20 to 50% investment savings). In addition, due to the simplicity, smaller sludge production and lower power consumption of the combined anaerobic/aerobic system, the operational costs also represent an even greater advantage. Savings on operation and maintenance costs are usually in the range of 40 to 50% in relation to a conventional treatment plant.  

Some of the main possible combinations of UASB reactors with post treatment systems are illustrated in the following figures. It can be observed that in the UASB + activated sludge and UASB + biofilm aerobic reactor systems, the aerobic biological excess sludge is simply returned to the UASB reactor, where it undergoes digestion and thickening with the anaerobic sludge, dispensing separate digestion and thickening units for the aerobic sludge. Hence, the overall excess sludge from the combined system is wasted only from the UASB reactor. Since it is already thickened and stabilised, and can be directly sent for dewatering and final disposal. Sludge drying beds have been frequently used in small-sized plants. Thus a large simplification in the overall flowsheet is obtained, including the liquid (sewage) and solid (sludge) phases.

Comparison between wastewater treatment processes

The availability of treatment technologies to be potentially applied for the treatment of urban wastewater is very large. The decision regarding the process to be adopted should be derived from a balance between technical and economical criteria, taking into account quantitative and qualitative aspects of each alternative. However, many aspects are frequently intangible and in a large number of situations, the final decision can still contain a level of subjectivity. Criteria or weightings can be attributed to the various aspects essentially associated with the local reality in focus, so that the selection really leads to the most adequate alternative for the system under analysis. There are no such generalised formulas for this, and the common sense and experience when attributing the relative importance of each technical aspect is essential. While the economic side is fundamental, it needs to be remembered that the best alternative is not always the one that simply presents the lowest cost in economic-financial studies.

Figure 5 presents a comparison between important aspects in the selection of treatment systems, analysed in terms of developed and developing regions (von Sperling, 1996). The comparison is unavoidably general, due to the specificity of each region or country and the high contrasts usually observed in developing regions. The items are organised in a decreasing order of importance for the developed regions. In these regions, the critical items are usually: efficiency, reliability, sludge disposal aspects and land requirements. In developing regions, these first items are organised in a similar manner of decreasing importance, but have a lower magnitude, in comparison with the developed regions. The main difference resides in what are considered the critical items for the developing regions: construction costs, sustainability, simplicity and operational costs. These items are of course important in developed regions, but cannot be usually considered critical.

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Fig 5. Critical and important aspects in the selection of wastewater treatment systems in developed and developing regions (von Sperling, 1996)

Tables 1 and 2 present a summary of typical characteristics of different variants of the main domestic wastewater treatment processes operating in warm climate regions (full description of the processes is given in the book). The large diversity can be clearly seen, and this wide variation is one of the strengths in the selection of the treatment technology to be adopted in a community. In each case, there will be one or more treatment processes that can satisfy the local requirements and maximise the benefits.

Table 1. Typical characteristics of domestic wastewater treatment processes operating in warm climate regions

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Notes

  • Chemical precipitation of phosphorus with any of the technologies above: P < 1 mg/l
  • Disinfection: e.g. chlorination, ozonisation, UV radiation. Barrier: e.g. membranes. Provided the disinfection/barrier process is compatible with the quality of the effluent from the preceding treatment: thermotolerant coliforms < 103 MPN/100ml; helminth eggs: variable
  • Advanced primary treatment: removal efficiencies  vary depending on the coagulant dosage
  • In compact aerated systems (e.g.: activated sludge, submerged aerated biofilters) or after treatment with a UASB reactor, aeration control allows a certain economy (not all the installed power is consume
  • Sludge production is presented on an yearly basis, but sludge removal frequency may vary, depending on the treatment process

Table 2. Relative evaluation of the main domestic sewage treatment systems (liquid phase)M7.jpg

Notes: the grading is only relative in  each column and is not generalised for all the items. The grading can vary widely with the local conditions

  • +++++ : most favourable              + : least favourable                     ++++, +++, ++: intermediate grades, in decreasing order   0 : zero effect             + / +++++: variable with the type of process, equipment, variant or design
  • UASB reactor + post-treatment: (a) post-treatment characteristics prevail; (b) UASB reactor characteristics prevail
  • O&M: operation and maintenance

Concluding remarks

The vast array of available processes for the treatment of wastewater should be seen as an incentive, allowing the selection of the most appropriate solution in technical and economical terms for each community or catchment area. For almost all combinations of requirements in terms of effluent quality, land availability, construction and running costs, mechanisation level, environmental impacts and operational simplicity, there will be one or more suitable treatment processes.

When conceiving, designing and operating a wastewater treatment plant in a warm region, the climatic specificity must be taken into account, in order to make the best use of the many favourable characteristics brought about by the higher temperatures. In a similar way, for a treatment plant in a developing region, the relevant aspects that lead to its sustainability must be judiciously incorporated. For a successful reversal of the prevailing dramatic status of water pollution in many countries of the world, a deep knowledge of the theory and practice of wastewater treatment is obviously required from the technical people. However, it should be borne in mind that technology alone cannot reverse this picture, and commitment, enthusiasm, organisation and persistence are also indispensable tools in the hands of those involved.

Resources

The material in this article is taken from the book, Biological wastewater treatment in warm climate regions (von Sperling and Chernicharo, 2005)The distinguishing feature of the book is the consideration of the fact that regional climate and developmental status play a very important role in the conception, design and operation of biological wastewater treatment.

The main purpose of the book is to present the technologies for urban wastewater treatment as applied to the specific condition of warm temperature, with the related implications in terms of design and operation. There is no strict definition for the range of temperatures that fall into this category, since the book always presents how to correct parameters, rates and coefficients for different temperatures. In this sense, subtropical and even temperate climate are also indirectly covered, although most of the focus lies on the tropical climate.

This article contains exerpts from the book, and all the treatment processes listed are covered in detail in the book, written by:

Marcos von Sperling & Carlos Augusto de Lemos Chernicharo

  • Department of Sanitary and Environmental Engineering
  • Federal University of Minas Gerais
  • Brazil
  • marcos@desa.ufmg.br

References

VON SPERLING, M. (1996). Comparison among the most frequently used systems for wastewater treatment in developing countries. Water Science and Technology, 33 (3). pp. 59-72

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