Financial support: This work was supported by the Faculty of Science, Prince of Songkla University, Thailand.

Keywords: biochemical oxygen demand(BOD), chemical oxygen demand(COD), purple non sulphur photosynthetic bacteria (PNSB), rubber sheet wastewater, single cell protein (SCP).

Thailand is the world's biggest producer and exporter of raw natural rubber. Rubber sheet manufacturing processes produce wastewater that contains both organic and inorganic matter. Some of the matter comes from the natural rubber and the rest from the chemicals used in processing, such as ammonia, formic acid, sodium metabisulphite and sodium sulphite. Some of these materials are toxic and the wastewater has to be treated to remove most of these compounds, in a cost effective process, before release. Treatment normally takes place in lagoons or oxidation ponds which then produce hydrogen sulphide, and the rotten-egg odour is a major problem of the system.

Purple non sulphur bacteria (PNSB) are metabolically versatile bacteria, naturally associated with the rubber wastewater treatment lagoons in southern Thailand, when they form purple blooms. PNSB can grow in the presence of a variety of organic compounds and in the presence of toxic compounds such as H₂S, in the absence or presence of oxygen, in the light or in the dark and over a range of temperatures and pH values. Their growth requirements are met by metabolizing organic compounds just like normal facultative anaerobes like E. coli but in the light their energy requirements are supplemented by an anaerobic photosynthetic reaction, which allows them to grow as well.

Some, even have a limited ability to metabolize reduced S-compounds that minimizes the production of H₂S. Although their metabolic activities can result in an effective wastewater treatment process, the physiology of these natural populations of PNSB in the wastewater lagoons, which differ from place to place, has not previously been investigated in detail.

92 cultures of the PNSB were isolated from 23 samples of wastewater obtained from rubber sheet manufacturing processes from various places in southern Thailand.

Wastewater samples were inoculated into the liquid selective G-5 medium of Ormerod et al. 1961 for enrichment, then plated onto the same medium solidified with 1.5% agar for selection and purification of single colonies. Culture conditions were anaerobic in the light, at room temperatures (28-32ºC) for 96 hrs. Selection of cultures that might be useful for the treatment of wastewater was achieved by first inoculating the above isolates into sterilized wastewater obtained from a latex rubber sheet process plant at Yang-Ngam in the Hat-Yai district of Songkhla province and incubating with microaerobic-light conditions for 72 hrs.

Cultures showing good growth were then inoculated into both untreated and sterilized wastewater both with and without supplementation with 0.10% yeast extract and those showing good growth in all media and able to compete with the indigenous population were selected for further study. Culturing was at 30ºC under microaerobic-light conditions for 72 hrs. One strain, DK6, which produced the best growth on unsupplemented, non sterile wastewater was chosen for further identification and characterization. The optimum pH (6.5-7.5) and temperature (30ºC) for the isolate DK6 were similar to the conditions found in lagoons of rubber sheet wastewater treatment systems i.e. a pH of 6.5-7.7 and temperatures between 28-34ºC. It was identified as being most closely allied to Rhodopseudomonas blastica. Optimum growth was achieved after supplementing the wastewater with 0.50% (NH₄)₂SO₄ and 1 mg/L nicotinic acid under conditions of microaerobic-light (3000 lux). This isolate DK6, growing as either a pure culture (sterilized wastewater) or in the presence of the indigenous bacteria (raw wastewater) produced a reduction of 90% of both chemical oxygen demand (COD) and biochemical oxygen demand (BOD) and a significant population of microbes. Although it had no ability to utilize H₂S it did grow as a photolithotroph with thiosulphate but any effect it might have had on the production of H₂S during the treatment process was not tested.

Most indigenous populations reduced the BOD and COD of the same wastewater by only 70% and 54%, respectively. Chemical analysis of the sedimented bacteria after treatment of the enriched wastewater showed that the protein content of the pure DK6 was 65.2% of the dry weight, and in mixed culture the protein content was 66.7%. Hence, this isolate may be used to inoculate wastewater, where it is an effective competitor with indigenous organisms, catalyses an effective treatment process and can also provide single cell protein (SCP) that could be tested for use as an animal feed supplement.