Sludge thickening by foam filtration

Arıtma çamurlarının bertarafı, atıksu arıtma tesislerinin işletilmesinde dikkat edilmesi gereken önemli problemlerden biri olmaya devam etmektedir. Birçok ülkede akuatik ortamın korunmasında büyük bir rol oynayan küçük ölçekli işletmelerde çamur bertarafının mekanik yöntemlerle verimli ve ekonomik bir şekilde gerçekleştirilmesi oldukça zordur. Pek çok küçük ölçekli tesis atık çamurlarını işleyememekte ve oluşan atık çamurlarını başka tesislere göndermektedir. Bu çalışmada, sünger filtre kullanımı ile küçük işletme tesislerinde sıvı-katı ayrımının (susuzlaştırma işleminin) sağlanması ve çamur yoğunlaştırılmasında yaygın olarak kullanılan cazibeli çamur yoğunlaştırıcı, çözünmüş hava ile yüzdürme vb. konvansiyonel mekanik ekipmanlarla elde edilen sonuçların karşılaştırılması amaçlanmıştır. Sünger ile çamur yoğunlaştırma, yavaş kum filtresine oldukça benzer nitelikte fakat kum yerine sünger malzemenin kullanıldığı bir filtrasyon sistemidir. Çalışmada kullanılan çamur haftalık olarak İstanbul Su ve Kanalizasyon İdaresi (İSKİ) Paşaköy İleri Biyolojik Arıtma Tesisi’nin çamur geri dönüş hattından temin edilmiştir. Çamur sisteme peristaltik bir pompa yardımı ile filtre üzerinden farklı debilerde beslenmiştir. Çamurun katı-sıvı ayrımında, katı kısım sünger filtrenin üzerinde bir katman (çamur keki) yaratmış, sıvı kısım ise alttan drene edilmiştir. Biyolojik çamurun başlangıçtaki katı madde konsantrasyonu yüzde 0.8 ile 1.6 arasında değişmiştir. Sünger ile yoğunlaştırma işleminden sonra elde edilen katı madde konsantrasyonun yüzde 4.1 ile 6.4 arasında olduğu gözlenmiştir. Drene edilen sıvı fazda ise kimyasal oksijen ihtiyacı (KOİ) ve toplam yüzebilen katı madde (TYKM) miktarı ölçülmüş ve yerel kanala deşarj standartları ile karşılaştırılmıştır. Anahtar Kelimeler: Çamur katı-sıvı ayrıştırması, çamur susuzlaştırması, sünger filtrasyonu, küçük ölçekli arıtma tesisleri.Sludge processing and disposal is one of the most essential and important steps in wastewater treatment, due to the huge amount of sludges (biosolids) produced as a residue of the biological wastewater treatment plants. Inappropriate sludge treatment causes to serious environmental problems. Thickening and dewatering are the common processes for sludge treatment; sludge is reduced or disposed directly or after insulation, etc. Considering the overall cost of wastewater treatment including biosolids treatment, the efficiency of the solid-liquid separation process is a key factor in wastewater treatment (Dentel, 2001). The cost of the dewatering step in municipal treatment plants, including conditioning agents, typically accounts for 30-50% of the annual operating costs (Mikkelsen ve Keiding, 2001). Sludge dewatering was also pointed out as one of the most expensive processes (Burris, 1979, Bruus vd., 1992). Therefore, many researchers (Krofta ve Wang, 1986; Tokunaga vd., 1986; Vesilind, 1995) focused on improving the efficiency of the dewatering and thickening system. In small-scale wastewater treatment facilities, it is difficult to carry out the sludge treatment on site. In many cases the excess sludge is transported to the other sludge processing facilities. Sludge volume reduction is an important issue in terms of transport costs and energy saving. It is then worthy to develop a sludge thickening and dewatering system specific for small-scale wastewater treatment facilities. Gravimetric thickening, dissolved air flotation and centrifugation are the most common processes used for sludge thickening; Gravimetric thickening is commonly used in wastewater and water treatment facilities because of the simple structure and low energy requirements of the apparatus. On the other hand, the gravimetric thickening process requires a large volume thickening tank and long operation period. In addition, the supernatant from the gravimetric thickening tank contains a high level of SS and returning to water treatment is a necessary process for small-scale facilities. Some other mechanical thickening processes may not be applicable for small-scale facilities due to their high costs and low working rates. Foam filtration is based on catching the solid particles in the pores of the filter and using these blocked pores to form a sludge layer over the filter medium and make the sludge itself a filter medium and percolate only water under the filter medium, then decrease the sludge volume. The pore sizes changes between 200 – 250 µ and they don’t follow a regular pattern in 10mm thick foam, this fact enhances solid caking. Also water can percolate from the pores easily. Two phenomena take place simultaneously during thickening. They are namely retention of solids in the foam and expulsion of water. This is affected by various parameters; the major factors being gradation of solids in the sludge, water content of sludge, and opening size of foam. Another important aspect that needs to be taken care during thickening is the permeability of the foam. Permeability of the media does not remain constant throughout the process. It decreases with time. Decrease in permeability of a media is the result of clogging of pores. Clogging is a function of gradation and therefore the decrease in permeability is also a function of gradation. This study introduce the foam filtration as a new liquid-solid separation technique for the small scale treatment plants instead of the conventional equipments like gravity sludge thickener and dissolved air flotation systems. The foam thickening is a filtration operation simulating the slow sand filtration except the sponge medium instead of sand medium. In this study, the biological sludge samples were taken from the return sludge line of Istanbul Paşaköy Advanced Biological Wastewater Treatment Plant. The sludges were fed to the filter from the top by using a peristaltic pump at different flow rates. Solid part of the sludge was retained on the surface of the filter forming a layer (sludge cake) at the top of the mesh and the liquid part was drained from the bottom. The initial solids concentrations of the biological sludge ranged from 0.8 to 1.6 percent before the thickening. The solid concentrations of the biological sludge samples thickened by foam filtration were found to be in a range of 4.1 to 6.4 percent. COD and TSS of the filtrates obtained from the foam filtration of sludge were also analyzed and compared with respect to the discharge standards given at the related Regulation. Keywords: Liquid-solid separation of sludges, sludge dewatering, foam filtration, small-scale treatment plants