WaterSubject

Sustainability has recently been accepted as a key concept of domestic wastewater management in developing countries. However, although significant advancements have been made so far in wastewater treatment technologies, it has been very difficult for these countries to implement such expensive facilities. As a result of this, it has been realized that decentralized, low-cost natural treatment systems utilizing locally available resources should be investigated as an alternative to the modern sophisticated treatment processes.

Domestic wastewater in Thailand has a low organic content compared with typical sewage described elsewhere; yet, this is a fundamental fact that seems not to have been fully appreciated by local engineers. Climatic conditions such as high temperatures and heavy rainfall have resulted in a further reduction in the organic content of the wastewater. This is due to decomposition and dilution, and the fact that tendency to follow western-style wastewater treatment practices in the urban areas of Thailand is rapidly increasing.

However, construction, operation and maintenance of such expensive facilities have imposed an unnecessary economic burden on the country. At the same time, most of the conventional wastewater treatment plants have been running somewhat below their design capacities. In this context, the authors’ opinion is that an in-depth assessment of the biological treatability of wastewater in Thailand is not only important, but essential.

This paper aims to reveal the true facts on domestic wastewater characteristics and treatment performance in Bangkok, Thailand. To this end, water qualities of two typical domestic wastewater treatment plants in Bangkok were compared, and simple bench-scale laboratory experiments were carried out to examine organic contents and their degradation kinetics. The possibilities of using natural treatment systems with waste source separation for sustainable domestic wastewater management in Thailand are also discussed.

Wastewater treatment scenario

Traditionally, Thai people use water (for hygiene purposes) after defecation instead of toilet paper. This tradition still continues except in modern apartments, flats and hotels where foreign influence is significant. Water is commonly used after defecation instead of toilet paper in other countries in southern and southeastern Asia as well, and this practice may even extend to the whole of the tropical monsoon region.

Traditionally, human excreta in Bangkok is flushed with water into a septic tank (or underground leaching cesspool), and the supernatant from the tank, alongside household greywater, is supposed to be treated in conventional wastewater treatment facilities, or discharged to surface water bodies such as ditches, canals and ponds.

The existing sewers in the Bangkok Metropolitan Region (BMR), along with the previously mentioned ditches, canals and ponds, were primarily intended for safe disposal of floodwater, and in spite of the modernization of collection and treatment facilities such as interceptor sewers and activated sludge reactors, the old box culvert sewers and leaching pits still continue to exist.

As a result of this, the domestic wastewater treatment facilities in Bangkok can be grouped into two types with respect to the organic strength in the influents to them. One type receives septic tank supernatant and household greywater, while the other receives direct toilet flush products in addition to the former two. ‘Sri Phraya’ (SP) and ‘Huay Kwang’ (HK) are typical sewage treatment plants representing these categories, respectively.

Wastewater

Bench-scale laboratory experiments were conducted with typical Thai domestic wastewater taken from the outlet of a sewer pipe in ‘Ruamrudee’, a residential area in the central BMR.

Wastewater quality surveys

Organic contents in terms of 5-day biochemical oxygen demand (BOD₅) and chemical oxygen demand (COD) in the influents to the SP and HK wastewater treatment plants were compared based on the data obtained from the plants. Nitrogen and coliform bacteria contents in the influents and effluents of these two treatment plants were also analysed and compared both in dry and rainy seasons. The sampling dates were selected based on the local weather conditions (dry and rainy seasons) in Bangkok.

Laboratory experiments

Three simple laboratory experiments were carried out to assess organic degradation kinetics and the additional effects of mixing, dilution and toilet paper addition on the microbial degradation of the domestic wastewater. All the experiments were carried out in a darkened room, and at a constant temperature of 30°C.

Organic degradation and the dilution effect

The wastewater samples were kept in two 20L capacity plastic containers. One container was mixed with a vertical stirrer (g=18.64/s, Re?12 700 within the turbulent range) while the other was not stirred at all. The latter represented a typical Thai situation in which the wastewater remains stagnant for quite some time in terminal sewers with gentle inclines. The blade of the stirrer, made from a piece of steel flat (7.7 × 2.5 cm), was constantly immersed in the water. The degradation dynamics were monitored by measuring selected parameters (BOD₅, COD, nitrogen, coliform counts) at specified time intervals over a period of 3 weeks. Similar experiments were conducted with (two-, three- and five-fold dilution of the model wastewater with distilled water), and without dilution as a reference marker. None of these samples was stirred during the experiment.

The following conclusions are drawn from the above simulated experiments:

These findings reveal hitherto unmentioned facts about the contradictions between the planning and design of wastewater treatment plants and the climatic and cultural conditions in Thailand. Based on this study, it is felt that further investigations and implementations are urgently needed in order to pursue the practice of sustainable management of wastewater, extremely relevant in tropical monsoon regions.