Evaluation of parameters which effect phosphorus release by using Box-Wilson experimental design method

Arıtma çamurlarında fosfor salınmasını etkileyen parametrelerin Box-Wilson deneysel tasarım metodu kullanılarak incelenmesi

Abstract

The usual forms of phosphorus found in aqueous solutions include the orthophosphate, polyphosphate, and organic phosphate (Metcalf & Eddy, 1991). Wastewater discharges of nitrogen and phosphorus to the environment are undesirable because these nutrients accelerate eutrophication (Lee et al., 2003). Therefore, various phosphorus removal technologies have been developed like chemical precipitation, biological phosphorus removal, ion exchange (Morse et al., 1998).Steen (1998) has pointed out that the annual global production of phosphate is around some 40 million tones of P2O5, derived from roughly 140 million tons of rock concentrate by the year 1998. In addition, World fertilizer use would reach 60-70 million t/a of phosphates by 2050. It is concluded that global phosphate resources extend, for all intents and purposes, well into the future, but that depletion of current economically exploitable reserves can be estimated at somewhere from 60 to 130 years (Steen, 1998). Phosphrus recovery from sewage sludge and sludge ash are applied at two steps. These are phosphorus release from sludge and recovery of released phosphorus from phosphrus-enriched liquid phase. In this study, nitric acid (inorganic acid) and oxalic acid (organic acid) were utilized in order to release the phosphorus fixed in the sewage sludge. The effects of parameters which determine the efficiency of mixture temperature, mixing time and acid concentration on the phosphorus release were investigated. For the analysis of the results, Box-Wilson Experimental Design Method has been used. Sludge samples taken from belt press stages of Çiğli municipal wastewater treatment plant were dried at 103ºC in oven. Chemical composition of dried sludge was determined using X-ray diffraction analysis (XRD-Rigaku). Dried sludge samples were leached with nitric acid and oxalic acid), by applying different concentrations, various mixture temperatures, and various mixing times which were adjusted to desired levels specified by the Box–Wilson Experimental Design Method. Magnetic stirrer/hot plate and electronic temperature controller with stainless steel temperature sensor were used for the leaching process. Leachate solution with different concentrations was added to the dried sludge samples and was mixed for various time periods at 300 rpm. Ratio of dried sludge (g)/ solution volume (ml) was chosen as 1/50. The concentration of phosphate in the leachate was analyzed using Stannous Chloride Method; 4500-P D according to Standard Methods (APHA, 2005). All leaching stages are repeated twice for each experimental step. Box–Wilson Experimental Design Method was used to determine the effects of parameters, such as mixture temperature (X1) solution concentration (X2) and mixing time (X3) on phosphorus release from dried sludge. Solution concentration was varied between 0.1- 4 M for nitric acid and 0.1-2 M for oxalic acid. In addition mixing time was between 15-180 min and mixture temperature was in the range at 20ºC and 100ºC. The experiments consisted of six axial (A), eight factorial (F) and one centre (C) points. The centre point was repeated three times. The amount of phosphorus release was described by using response function. Statistica 5.0 software program was used to determine, the response function coefficients. The predicted values of phosphorus release amount were in accordance with the observed values. It was found that R-squared values are 0.995, and 0.853 for nitric acid, oxalic acid respectively. Results were evaluated in terms of the effects of concentration-mixing time, temperature-mixing time, and concentration-temperature on phosphorus release from the sludge.The analysis results have shown that nitric acid is more efficient as compared to oxalic acid. In experiments carried out with nitric acid, it has been observed that the increase in nitric acid concentration and mixing time do not significantly affect phosphorus release, while an increase in mixing temperature effects considerably. Experiments conducted using oxalic acid have shown that increasing mixing temperature results increasing phosphorus release, whereas concentration increase and mixing time have no significant effect. The best phosphate release amounts were accomplished, at 4 M concentration, 100ºC mixture temperature, and 97.5 min mixing time for nitric acid. In experiments with oxalic acids, The best phosphate release amounts were accomplished at 2 M concentration, 100ºC mixture temperature, and 97.5 min mixing time.

Özet

Son yıllarda, fosfor doğal rezervlerinin hızla tükenmesi nedeniyle birçok endüstriyel sektör için ana ham madde kaynağı olan fosforun atıksu ve arıtma çamurlarından geri kazanılmasına yönelik yapılan araştırmalar önem kazanmıştır. Arıtma çamuru ve çamur küllerinden fosfor geri kazanımı için; çamur bünyesinden salınması ve sıvı ortama alınan salınmış fosforun geri kazanılması olarak iki aşamalı yöntem izlenmektedir. Atıksu ve arıtma çamuru bünyesinden fosforun salınması amacıyla çeşitli kimyasal ve ısıl işlemler kullanılmaktadır. Bu çalışmada, arıtma çamuru bünyesinde bağlı olan fosforun salınması için asidik işlem uygulanmış; bu amaçla inorganik bir asit olan nitrik asit ve organik bir asit olan oksalik asit kullanılmıştır. Fosfor salınması üzerinde karıştırma sıcaklığı, karıştırma zamanı ve asidin konsantrasyon değeri gibi parametrelerin etkileri araştırılmıştır. Sonuçların değerlendirilmesi için Box-Wilson deneysel tasarım metodu uygulanarak salınan fosfor miktarı için hesaplanan değerler ile deneysel olarak ölçülmüş değerler arasında uyum olduğu belirlenmiştir. R2 değeri nitrik asit için 0.995, oksalik asit için 0.853 olarak bulunmuştur. Değerlendirme sonuçlarına göre; fosfor salınmasında nitrik asitin oksalik aside göre daha etkili bir kimyasal olduğu belirlenmiştir. Nitrik asitle yapılan çalışmalarda, konsantrasyon ve karıştırma zamanına göre sıcaklığın fosfor salınması miktarını önemli ölçüde arttırdığı gözlenmiştir. Oksalik asitle yapılan çalışmalarda nitrik asitle yapılan çalışmalarda olduğu gibi sıcaklığın daha etkili bir parametre olduğu gözlenmiştir. En yüksek fosfor salınım miktarı, nitrik asitle çalışıldığında 100ºC karıştırma sıcaklığı, 4 M konsantrasyonda ve 97.5 dakika karıştırma süresi kullanılarak; oksalik asitle yapılan çalışmalarda ise 100ºC karıştırma sıcaklığında, 2 M konsantrasyonda ve 97.5 dakika karıştırma süresi ile elde edilebileceği belirlenmiştir.