Anaerobik çürütülmüş çamurların farklı kimyasal maddelerle şartlandırılması

Abstract

Susuzlaştırma uygulamaları açısından çamurların suyunu salma kabiliyetlerinin arttırılması önemli bir husustur. Bu çalışmada, Ataköy Biyolojik Atıksu Arıtma tesisi'nden alman anaerobik çürümüş çamur numuneleriyle yapılan deneylerde bu tür çamurlarla, farklı şartlandıncı maddeler ve değişken karıştırma şiddetleri altında uygun süzülebilirlik koşullarını sağlayıp sağlamayacağı tespit edilmeye çalışılmıştır. Çamur numuneleri yoğunlaştırmadan önce ve sonra olmak üzere iki grupta kullanılmışlardır. Deneylerde üç grup şartlandıncı madde kullanılmıştır. Bunlar ; Polielektrolit, FeCb ve Ca(OH)2. Deneylerde şartlandıncı madde ile çamurun yeterli karışımını sağlamak için 100 devir/dk'hk karıştırma şiddeti uygulanmıştır. Şartlandırma sonrasındaki çamurun bir sonraki kademeye iletilmesindeki parçalanmayı temsil etmek üzere çamurlar 1000 devir/dk şiddetindeki karıştırıcıda farklı sürelerde parçalanmaya tabi tutulmuşlardır. Parçalanma sonrasında ise yeniden yumak oluşumunu incelenmek üzere tekrar 100 devir/dk'lık kanştırma şiddeti uygulanmıştır. Çalışmanın sonuçlarına göre, optimum koşullar sağlanmak şartıyla daha iyi susuzlaştırma verimleri elde edilebileceği tespit edilmiştir. Ayrıca, deneylerde şartlandırmanın supernatant kalitesine olan etkisi de incelenmiştir. Bunun sonucunda ise, optimum şartlandırma koşullarında kirletici konsantrasyonlan (TKN, KOI, TP) daha düşük bir supernatant elde edileceği, bunun da arıtma tesisine gelen toplam kirlilik yükünde kayda değer bir azalma sağlayacağı tespit edilmiştir.

The term sludge conditioning must be understood as the overall process of enhancing the aggregation of suspended solids by chemical and/or physical methods. Conditioning of sludges neutralizes the chemical or physical forces acting on suspended sludge particles in a liquid (e.g. water). This neutralization process brings about the growth of the otherwise small visible sized particles into larger aggregates known as floes. These floes have an indefinite shape with random and noncrystalline structure. The use of chemicals to conditioning of sludges for dewatering processes is economical because of the increased yields and greater flexibility obtained. Conditioning of sludges can reduce the 90-99 % effluent sludge moisture content to 65-85 %, depending on the nature and source of the sludge. The purpose of such enhancement of floe growth or conditioning is in general to increase the efficiency of dewatering or thickening of slugde through various treatment processes intended to concentrate the sludge solids. It can be more understood why it is so important, if it is considered that the cost of sludge treatment in a plant is equal to 30-40 % of total treatment costs. Estimating expected sludge quantities is not a problem for engineers and operators of existing wastewater treatment plants. For plant expansions and for brand new plants, it is often great importance to correctly predict the quantities and characteristics of sludge produced. Some problems in plant operations and/or too big plant designs must be expected if the quantity of sludges produced cannot be estimated correctly. In general, there are two mechanisms of chemical conditioning. The first is the neutralization of particle charge and the second is particle bridging. These mechanisms are distinguished by the terms of coagulation and flocculation, respectively. Coagulation involves destabilization of the sludge particle by decreasing the magnitude of the repulsive electrostatic forces between particles in sludge liquor. This process occurs through compression of the electrical double layer surroundig each particle. Flocculation follows coagulation and is the agglomeration of colloidal and finely divided suspended solids gentle mixing. In addition to destabilizing surface electrical charges on dispersed sludge particles, polyelectrolytes flocculate sludge particles during the bridging process. Bridging is the simultaneous attachment of polymer molecules, which disperse as XV a along chain in a sludge liquor, to two or more sludge particles. In this way, polymers fill the medium between sludge particles and draw them together in a lattice structure to form floes. Sludge conditioning by heat treatment or freeze/thaw cycles changes the surface properties of suspended solids and destroy the cells of biomass. Biological sludges contain water and cellular matter that exist in a gel-like form. When stresses, for example pumping forces, are acting on sludge particles the gel-like stucture of biomass breaks down and lysis also occurs. Many physical factors affect sludge conditioning, which is an important step before dewatering of sludges. Perhaps the most significant factor that effect the conditioning process is the particle size and distribution of sludge suspended solids. Studies on sludge conditioning show that the most important particulate group in sludge liquor is the 1-100 urn sized group which called supracolloid. The greater the supracolloid concentration the poorer dewaterability and settleability is obtained. Sludge origins and nature of the solids matrix from a certain unit process also influence sludge conditioning. For example, primary sludge requires less conditioning than secondary sludge (e.g. waste activated sludge). The degree of mixing intensity in the preparation of solutions of organic coditionin agents is an important factor because high itensity mixing tends to cut or destroy long polymer chains in to shorter, less effective fragments. The most important chemical factors influencing conditioning of sludges are the alkalinity and pH. The charge densities on the surface of the suspended sludge particlesand on a conditioning agent are functions of pH. Alkalinity is an important sludge property that modifies conditioning requirements and the pH of sludge affects alkalinity. High alkalinities tend to have detrimental effects on the performance of both inorganic and organic conditioning agents. And tend to increase the dosage requirements of a conditioning agent. The charge density and molecular weight of a conditioning agent govern its performance. In general, as the charge density and molecular weight of conditioning agent increase, so does its effectiveness in forming good floes. The concentration of a conditioning agent is an important factor in the dewatering of sludges and is related to the viscosity of its solution. Water used for dilluting the conditioner may often be applied close to the mixing point of the sludge and conditioner, in order to obtain effective distribution of the conditioner. In general, biological sludges have different conditioning requirements than raw and primary sludges. Anaerobically digested sludges tend to have lower requirements of cationic conditioning agents than aerobically digested sludges. Cationic polymers, in general, are the most effective conditioning agents in conditioning biological sludges. Because of this, in the experimental studies we use caionic polimer solutions to obtain effective results. Since a long time both chemical and physical methods are in use to improve dewaterability of sludges. Recently, however, organic conditioners such as polymers have begun to replace inorganic conditioners in many sludge dewatering processes. XVI The two most commonly used inorganic conditioning agents are lime and liquid ferric chloride. Liquid ferrous sulfate, anhydrous ferric chloride, aluminum sulfate, aluminum chloride and potassium permanganate are also in use, although less frequently. Iron and aluminum salts dissociate to form divalent or trivalent metal ions that react with hidroxyl and other alkaline ions. The hydroxides of these metals are very insoluble and precipitate rapidly. The widely used backbone monomer is acrylamid. When polymerized, polyacrylamidis formed, which is a long-chain molecule with molecular weight in millionsand which is essentially nonionic. Examples of typical cationic polymers include: polyamines, polyethylimines, polymidamines, polybutadiens, polyamidamines, polyquarternaries and substituted polyacrylamides. Nonionic polymers incude: polyglyxydil polimers and polyacrylamides. Examples of typical anionic polymers include: polyacrylates, carboxylic polymers, and substituted polyacrylamide. Perhaps the most important and difficult problem in wastewater treatment is the treatment and dewatering of slugdes from different parts of the plant. One of sludge dewatering's most bothersome aspects is that there seems to be no accepted means to evaluate the ease with which a sludge will release its water. The first widely used sludge characterization technique was the specific resistance to filtration (SRF) test which was developed by Coackley and Jones (1956). Gale and Baskerville (1968) developed an apparatus which they called capillary suction time (CST) apparatus. This experimental method was heralded as a quick and easy method for determining sludge dewaterability. The good side of this technique is that it does not require skilled technicians. But unfortunately. CST test is an empirical test and is not based on a theoretical analysis. Then it would be very useful if CST could be mathematically developed. The purpose of the experimental studies were to evaluate and determine the effects of sludge characteristics on sludge dewaterability. Sludges were taken from the anaerobic sludge digester of Ataköy Biological Wastewater Treatment Plant. A CST apparatus was used to evaluate the filterability of sludges when adding polyelectrolytes as conditioning agents. Some important adverse effects can occur when the supernatant is recycled to the head of the treatment process. To evalute the effects of polymer conditioning some experiments (COD, TKN, TP) were done on supernatant quality. Four different polymers were used in sludge conditioning studies (CYTEC A100, CYTEC C200, CYTEC C250 and PRAESTOL 630 BC). In addition to this chemicals Ca(OH) and FeCk were also used. Experimental studies were divided into three groups : 1. Thickened digested sludge 2. Supernatant of digested sludge 3. Raw digested sludge XVII A mixing intensity of 100 rpm was used to provide an adequate mixing of the conditioner and the sludge. To represent the deterioration after the first conditioning of sludges a mixing intensity of 1000 rpm was applied. To examine the reflocculation event after deterioration, the 100 rpm mixing intensity was used again. The best results were obtained by the polyelectrolyte named PRAESTOL 630 BC. It has been observed that optimum dosage and CST value is 3 mg/gr S S and about 12 sec, respectively. Results of this study showed that, if optimum conditions could be provided, better dewatering performances could be achieved. Furthermore, in this study, the effect of conditioning on supernatant was experienced too. It is showed that under optimized conditions lower pollution concentrations in supernatant could be achieved.