The effect of stabilized wastewater sludge on nitrogen and urease activity in a soil

Bu çalışmada, farklı yöntemlerle stabilize edilen çamurların uygulandığı topraklarda çeşitli azot formları ve üreaz aktivitesi değerlerinde meydana gelen değişimler belirlenmiştir. Evsel nitelikli ham arıtma çamuruna, patojen popülasyonunu değişen derecelerde azaltan dört farklı stabilizasyon yöntemi (havada kurutma, pastörizasyon, kireç stabilizasyonu ve kireç+kül stabilizasyonu) uygulanmıştır. Stabilize edilen arıtma çamurları 50 ve 100 ton ham kuru çamur ha-1 oranını sağlayacak şekilde 2 tekrarlamalı tesadüfi blok tasarımı düzeninde 500 g kuru toprak içeren kaplara eklenmiştir. Örnekler 28ºC’de 34 gün boyunca inkübe edilmiştir ve inkübasyonun 5, 8, 15, 22, 29 ve 34. günlerinde alınan örneklerde toplam azot, amonyum azotu ve nitrat azotu konsantrasyonları ile üreaz aktivitesi seviyeleri belirlenmiştir. Uygulanan havada kurutma, pastörizasyon, kireç stabilizasyonu ve kireç+kül stabilizasyonu yöntemleri, çamurun fekal koliform içeriğini önemli derecede (4.12 ila 7.74 log) azaltmıştır. Uygulanan stabilizasyon yöntemleri USEPA kriterleri çerçevesinde değerlendirildiğinde, kurutma, kireç stabilizasyonu ve kireç+kül stabilizasyonu yöntemlerinin B sınıfı biyokatı oluşturduğu sonucuna varılmıştır. Diğer yandan, çamurun pastörize edilmesi (70ºC’de 30 dk.) ile USEPA A sınıfı biyokatı kriterleri sağlanmıştır. Sonuçlar çamura uygulanan stabilizasyon yöntemlerinin arasındaki farkların, toprakların toplam azot konsantrasyonu ve üreaz aktivitesi değerine olan etkisinin önemsiz olduğunu göstermiştir. Diğer taraftan, çamur uygulanan topraklarda belirlenen amonyum ve nitrat azotu değerlerinin ise çamura uygulanan stabilizasyon yöntemlerine bağlı olarak değiştiği tespit edilmiştir. İnkübasyon sonuçları özellikle alkali çamur uygulanan topraklarda, nitrifikasyon prosesinin amonifikasyona göre daha hızlı yürüdüğünü göstermiştir. Anahtar Kelimeler: Arıtma çamuru, azot formları, çamur stabilizasyonu, üreaz aktivitesi, toprak.Wastewater sludge has been already utilized in agricultural applications for several years as it represents an alternative source of nutrients for plant growth and an efficient soil conditioner enhancing certain physical properties of soil. However, wastewater sludge may contain pathogenic organisms and pollutants. This may negatively affect the soil properties. Recently increased attention was paid to the sludge stabilization process aiming to reduce the microbial content of sludge (pathogens). In this context several methods, such as biological digestion, composting, lime stabilisation, heat treatment have been used to eliminate pathogens from sludge. In this research, variations of various nitrogen forms and urease activity values have been determined in soils amended with wastewater sludges treated with different stabilization methods. Raw domestic sludge samples were treated with four different stabilization processes (air drying, pasteurization, lime stabilization and lime+fly ash stabilization) for reducing pathogen population with varying efficiency. Details of the processes are given below: air drying process: sludge was dried at 35°C by using a supplemental heat source; pasteurization: sludge was heated at 70°C for 30 minutes; lime stabilization: sludge was mixed with 15% of lime(CaO) on dry-weight basis; lime+fly ash stabilization: lime (10%)-fly ash (40%) mixture was added to sludge to increase the pH to 12. Lime is considered as one of the most common amendment materials for sewage sludge stabilization, as it plays significant role in reducing the microbial content of sludge (pathogens), enhancing the agricultural benefits and lowering the respective environmental risks. Fly ash, i.e. the by-product produced from fossil fuel thermal power plants, may contain increased amounts of calcium and magnesium oxides, depending upon the coal sources and may present highly alkaline values similar to lime. Thus, fly ash can be used as an alternative material for sludge stabilization with additional benefits, such as the reduced purchasing cost, and the minimization of fly ash disposal cost. Furthermore, the combined addition of fly ash and lime in small dosages to sewage sludge may result to an efficient removal of pathogens. In this research, de-watered sludge sample was collected from the treatment plant of canned food industry. The fly ash used in this study was obtained from Orhaneli Power Station where lignite is used for fuel. Technical grade anhydrous calcium oxide (quicklime 96%) was used together with fly ash in alkaline stabilisation. Stabilized sludge samples were amended to soil pots (500 g air dried soil) at rate equivalent to 50 and 100 tons dry sludge ha-1 and the pots were then arranged in a randomized block design with two replicates. The samples were incubated at 28°C for 34 days and total-N, ammonium-N and nitrate-N concentrations and urease activity levels were determined at 5, 8, 15, 22, 29 and 34 days of incubation period. Fecal coliform numbers were determined by Most Probable Number Method of Standard Methods. Concentrations of nitrate-N and ammonium-N were determined in samples which were extracted using 2 M KCI. The concentrations in extracts were analyzed by steam distillation with MgO and devarda alloy. Total N contents were measured by Kjeldahl digestion method. The urease activity of the soil was determined as described by Tabatabai. Applied methods of air drying, pasteurization, lime stabilization and lime+fly ash stabilization significantly decreased (4.12 to 7.74 log) the fecal coliform contents of wastewater sludge. When applied stabilization methods were evaluated by USEPA criteria, it can be concluded that air drying, lime stabilization and lime+fly ash stabilization generated Class B biosolid products. On the other hand, Class A biosolid requirements were achieved by pasteurization of sludge(70°C for 30 min.). The results indicated that the effects of applied sludge stabilization methods on total N concentration and urease activity values of soils were found to be insignificant. However, the ammonium and nitrate nitrogen concentrations in sludge amended soils were dependent on the applied sludge stabilization methods. In general, low levels of ammonium and high levels of nitrate nitrogen were determined in sludge amended soils during the incubation period. The results of the incubation indicated that nitrification proceeded much faster than ammonification especially in soils amended with alkaline sludges. Keywords: Nitrogen forms, sludge stabilization, soil, urease activity, wastewater sludge