Effluent treatment using the CSPR system

Author Koentoro Soebijarso is currently a lecturer on industrial engineering at the technical faculty of the University of Sarjanawiyata Tamansiswa Yogyakarta. In the past, he has served as director of IRDLAI (Institute for Research and Development of Leather and Allied Industry), Yogyakarta; president of the ISLTC; and dean of the technical faculty of the University of Sarjanawiyata Tamansiswa Yogyakarta (1997-2000). The conversion of hides or skins to make leather is water and chemicals intensive and has been known to cause serious environmental pollution problems. In 1995, the government of Indonesia issued a quality standard for wastewater produced by industrial processing industries. For the tanners, quite often located in urban areas and usually near a river, the imposition of environmental legislation meant they were urged to construct a suitable effluent treatment plant within the limited area of land available. This paper describes an experimental effluent wastewater treatment plant for a medium-scale tannery located in an urban area. The CSPR (continuous stirred pit reactor) system was chosen with the ARP (activity relation plant) model, with the emphasis on chrome tanning. Day-to-day operation of the system has proved reliable and fulfils the effluent standards of a tannery. Introduction The leather and leather products industries of Indonesia are spread across almost the whole of the country. Leather production is both a science and an art and tanning is the process of converting the unfixed animal skin into a stable product. In a wider sense, the term tanning in general covers processes in which proteins are stabilised in industrial as well as biological processes (Gustavson 1956). A tannery with an output of 100,000 sq ft of finished leathers/month is classified as a medium scale tannery. This type of tannery converts hides or skins into semi or finished leathers for which there are several methods: vegetable, chrome, synthetic etc. I should like to discuss only chrome tanning and, more specifically, effluent treatment using the CSPR system. In 1893 Martin Dennis patented a process of tanning skins directly in a solution of basic chromic chloride, later known as the one-bath process, which produced better leathers. Wastewater quality standard According to ministerial decree, the quality standards for wastewater, especially tannery wastewater, is: Parameters (mg/l) Limits pH6-9 BOD⁵ 150 max COD 300 max TTS 150 max H₂S 1 max NH₃H 10 max Cr total 2 max Oil and fat 5 max All of the eight parameters must be fulfilled by the tannery before the waste effluent can enter the sewerage system. The governor of the Yogyakarta Special Region was concerned about the environmental cleanliness and cleaner production system and decided to relocate all the tanneries to one area, with the support of the IRDLAI and the UNDP. This is now under construction. In October 1995, the government launched the decree for the quality standards of wastewater and this was followed by the issuance of the clean river conservation scheme which is valid for all process industries (Decree No 51/MENKLH/10/1995). Historically, many of the tanners were located in or near large towns such as Jakarta, Surabaya, Yogyakarta etc. This did not take into account the future modernisation and expansion of the big cities which came years after the original construction of the tanneries. A huge constraint is the limitation of land availability and the high cost of the land needed for the treatment plant. Many of the tanneries are situated long distances from central wastewater treatment facilities provided by regional governments which means that their only solution is to build their own in-house treatment plant. They need to continue production to fulfil contracts for semi and finished leathers for both the domestic and international markets. The smaller tanners concentrate away from big cities, in clusters such as Garut, Magetan, Pakalongan, and so face the same problems. From a chemical point of view expressed by Sandro Silva, 1989, chromium III, unlike chromium VI which is toxic, tends to shift in soil where it is absorbed by plants and vegetables through an active process. The fixation of chromium with the collagen protein during chrome tanning was highlighted by Gustavson, 1956. The chromium complex structure forms a crosslink between the protein chains and forms a multi-linking compound which is shown in the following equation: SO₄^- + H₃ N.P(COO Cr₃ COO)P.NH₃ + SO₄^- 2 The main characteristics of tanning are water, chemicals, labour and capital machinery with abundant amounts of water going into the soaking, liming, deliming, pickling and tanning processes. Consequently, the effluent contains all the chemicals used in the conversion processes, including chrome. Water is indispensable and used for drinking, washing, as a source of power, transportation, industrial processing etc and it is important to keep rivers as clean as possible. A cleaner production system will also help keep rivers clean and wastewater treatment is a sub-system of cleaner production as a whole. Design considerations The operational characteristic of a medium-scale tannery is the intermittent production system which depends on orders given by a third party and market demand for leathers. The requirement is to construct a wastewater treatment plant to handle intermittent process operations on the limited land available, the variability and volume of wastewater and maintain the sustainable production system with emphasis on cleaner and environmentally-friendly production. As stated before, the wet processes use the greatest volume of water and they are also chemical and capital intensive. Some of the chemicals used for the process stages in the lime yard and tan yard, based on kg/100kg of raw material, are shown in Table 1. As an illustration, the amount of water used for producing jacket leather from wet-salted sheepskins (200 pieces), water consumption for tanning is around 13,800 litres (Table 2). According to the type of production for a medium-scale tannery and 100,000 sq ft of leather monthly, equal to about 37 tons of raw sheep or goat skins a month, the tannery needs around 3,700kg chrome salts Cr₂(SO₄)₃ for the production of a leather jacket (sheep). Other chemicals can be calculated using Table 1. Operating the CSPR Operating the CSPR-based job order for a medium scale tannery with a construction of 100m³ wastewater (Figure 1), the following steps must be carried out consecutively: * Fill the CSPR * Start stirring immediately * Add calcium hydroxide solution * Add ferrous sulfate crystals, add flocculent * Ensure that the formation of multi-charge-complex-ion is formed, a black-blue precipitation * Stop stirring * With the aid of a submersible pump, allow the clear liquid to flow to the biological treatment using the pipe from the service stages (arrow Fig. 1) * With the same submersible pump, allow the sludge settled on the bottom of the CSPR I to flow to the rapid sand filter for drying purposes, through pipe direction of the cleaned processed effluent to the sewer Since tannery production needs to be carried out continuously and the effluent directly processed, the construction of the CSPR should have two or three inlets to allow for the filling operation without causing a stoppage of production. An effective layout of all the facilities for the ARP is provided in Figure 2. Experimental results Experimental results with the CSPR system wastewater treatment plant located in a urban city are shown in Table 3. Using the CSPR method, followed by a biological system, reduces effluent treatment parameters to less than the government regulations. Conclusions The CSPR and activity relationship is well suited to a medium-sized tannery effluent treatment plant operating with an output of 100,000 sq ft of leather monthly. The final results conform to the government standards of wastewater effluent for tanneries (Table 3). This is thanks to the long-term programme to relocate tanneries to the designated Yogyakarta Special Region. Acknowledgments The author wishes to thank the director and staff of the IRDLAI and the manager of the Bintang Buana Sakti leather factory for the experiments and observations. Also, the dean of the technical faculty of the University of Sarjanawiyata Tamansiswa. The author would also like to thank his wife for the support given during the making of this paper.