The effects of Floc Disintegration Mechanisms on Sludge Dewatering Properties: Enzymatic Treatment of Sludges

Çamur stabilizasyonu, organik madde içeriğinin azaltılması, patojen organizmaların giderilmesi ve koku potansiyelinin azaltılması amacıyla çamura uygulanan birim işlemlerdendir. En yaygın kullanılan biyolojik stabilizasyon yöntemleri, anaerobik çürütme ve aerobik çürütmedir. Bu yöntemler, bir çok avantajlara sahip olmakla birlikte, stabilizasyon sonrasında elde edilen çamur zayıf su verme özelliklerine sahiptir. Bu da mekanik su alma işlemleri sırasında daha fazla şartlandırıcı kimyasal madde tüketimi ve çamur kekinde düşük katı madde içeriği ile birlikte daha yüksek işletme ve bertaraf etme maliyetlerine neden olur. Atıksu arıtma tesislerinde oluşan arıtma çamurlarının suyunu güç vermesindeki biyolojik nedenler üzerine son dönemde yapılan birçok araştırma mevcuttur. Anaerobik ve aerobik çürütme  proseslerinde, azalan enzim aktivitesinin sonucu olarak, katı madde indirgenme hızlarındaki düşüş ve flok yapıdaki farklılaşma çamurun su verme özelliklerini zayıflatmaktadır. Çamurların su verme özelliklerinden sorumlu olan çamur bünyesindeki protein, polisakkaritler gibi hücre dışı polimerik bileşenler; aerobik veya anaerobik ortamda hidrolizi gerçekleştirecek olan enzimlerin aktivitelerindeki azalmayla birlikte bozunamayıp, ortamda birikmeye başlamaktadır. Aerobik çürüme işleminde polisakkarit miktarında gözlenen artış; anaerobik çürüme işleminde ise hem protein hem de polisakkarit bozunmasındaki azalmaya rağmen bunların mertebe olarak aerobik çürüme işlemindeki değerlerden daha yüksek olması, azalan enzim aktivitesinin en önemli göstergesidir. Bu makale kapsamında, arıtma çamurlarının enzimlerle arıtımı,  enzimatik arıtımın flok ayrıştırma mekanizmaları ve oluşan çamur miktarı üzerindeki etkilerinin incelendiği araştırmaların sonuçları verilmektedir. Ayrıca enzimlerin çamurların su verme özelliklerini nasıl etkilediği konusu irdelenmiştir. Anahtar Kelimeler: Çamur miktarının azaltımı, çürük çamur, enzimatik arıtma, flok ayrıştırma mekanizmaları, hücre dışı polimerik bileşenler (EPS), su verme özellikleri.Sludge stabilization process is one of the most important processes in sludge management, providing effective pathogen destruction, reduction of volatile solids and odor potential. Among different stabilization methods, biological stabilization - anaerobic and aerobic digestion- processes are more commonly used. Although the processes have many advantages, they may lead to poorer dewatering characteristics, which mean higher chemical requirements in sludge conditioning, lower quality of final processed biosolids, and higher operation and disposal costs. There have been many researches on the biological causes of strong water retention in wastewater sludges, particularly in waste activated sludges. Evidence from many researchers (Goodwin and Forester, 1985; Novak et al., 2003; Houghton and Stephensen, 2002) establishes that Extracellular Polymeric Substances (EPS), play a significant role in this regard. The EPS constituents act as a network that confines extracellular enzymes exhibiting hydrolytic activity. It was scientifically evidenced that both the concentration of hydrolytic enzymes, and the contact between these enzymes and their substrates, are very important particularly in the anaerobic digestion of complex particulate substrates (Jain et al., 1992). During both anaerobic and aerobic digestion, enzyme activity declines. It was reported that for aerobic digestion, the loss of the enzyme activity explains why polysaccharide accumulates in aerobic digestion process; for anaerobic conditions, the enzymatic activity for both protein and polysaccharide degradation decline, but remained higher than aerobic digestion. Sludge minimization strategies are receiving great attention recently to solve the sludge related problems in wastewater treatment plants which response lower investment and operational costs, and higher efficiency in the subsequent treatment processes. Thermal treatment, acidic or alkali chemical treatment, freeze-thawing, mechanical disintegration using ultrasonic devices, advanced oxidation processes like ozonation, and biological hydrolysis with enzymatic treatment are the mostly applied methods that have been introduced for sludge minimization purpose (Ayol et al. 2007). Although many strategies on sludge minimization have been reported in previous research studies (Wei et al. 2003, Liu and Tay 2001, Chu et al. 2001), biological hydrolysis using hydrolytic enzymes in stabilization units appears to be an effective and cost reducing process since it is at present on the chain of the sludge treatment. The term 'disintegration' is widely used in sludge minimization studies. Principally, it can be described as the destruction of sludge structure by external -physical, chemical, and biological- forces (Müller et al. 2004). Disintegration for biological processes incorporates the mechanisms: - cell lysis transforming cell content into the medium, - breakdown of extracellular polymeric substances (EPS) in the sludge flocculated matrix, and - the biodegradation of the end products by microbial metabolism. Preliminary researches on biological sludge disintegration (Wawrzynczyk et al., 2003, Ayol, 2005) showed that the enzyme additions to the sludges enhanced the solubilization of the particulate organic matters in the sludge. Ayol et al. (2007) have reported that the enzymatic sludge disintegration improved the performance of the aerobic and anaerobic pilot-scale digesters. The anaerobic digestion of the enzyme added reactor sludge resulted in a significant increase of methane production, and the EPS degradation compared to control reactor sludge. Previous research results indicated that if the hydrolysis of the complex organic structures in sludge with the hydrolytic enzymes like glucosidases, lipases, and proteases can be managed, it would be a great advantage for the efficient sludge minimization during biological sludge stabilization (Novak et al. 2003, Ayol, 2005.a). This paper reviews the enzymatic treatment of sludges and the effects of the treatment on floc disintegration mechanisms and sludge minimization. In addition to this, some research results on the role of enzymatic treatment on sludge dewatering properties were discussed. Keywords: Sludge minimization, digested sludge, enzymatic treatment, floc disintegration mechanisms, Extracellular Polymeric Substances (EPS), and dewatering properties