Precipitation of magnesium ammo-nium phosphate from source-separated urine

Kaynağında ayrılmış ve ayrı toplanmış insan idrarının tarımsal alanlarda gübre olarak kullanılması son yıllarda önem kazanmıştır. Evsel atıksu içinde hacimsel olarak %1’in altında bulunmasına rağmen, evsel atıksudaki azotun %80’i ve fosforun %60’ı idrardan gelmektedir. İdrarın kaynağında ayrılması ve depolanarak tarımsal alanlara uygulanması yararlı bir sistem gibi görünse de sistemin bir takım zorlukları ve riskleri mevcuttur. Bu çalışmada öncelikle idrar kompozisyonunun depolama süresince değişimi incelenmiş, daha sonra idrarın kaynağında ayrılması, depolanması ve tarımsal alanlara uygulanması sırasında karşılaşılan zorlukları azaltmak için idrardaki NH3-N ve PO43--P, magnezyum amonyum fosfat hekzahidrat (MAF) formuna dönüştürülmüştür. 4/1 oranında seyrelmiş olan idrardaki NH3-N molar konsantrasyonu 6 haftalık depolama süresi sonunda 0.009’dan 0.08 mol/l’ye yükselmiştir. pH 9.5 ve 10’da gerçekleştirilen çöktürme reaksiyonlarında idrardaki NH3-N’unun %87’sinin reaksiyonda yer aldığı saptanmıştır. Minimum MAF çözünürlüğünün gerçekleştiği pH olan pH 10.7’de ise NH3-N geri kazanımı %85.8’e düşmüş, buna karşılık PO43--P ve Mg’un geri kazanılma yüzdeleri sabit kalmıştır (sırasıyla %99.3 ve %99.7). Ayrıca, reaksiyona giren  NH3-N, PO43--P ve Mg’un başlangıç molar konsantrasyon oranlarının reaksiyon verimini etkilediği, magnezyumun molar konsantrasyonunun diğer reaktanlara kıyasla fazla olması durumunda NH3-N ve PO43--P geri kazanımının arttığı saptanmıştır. İdrardaki NH3-N’un başlangıç konsantrasyonunun diğer reaktanlara göre fazla olması durumunda elde edilen çökeltinin MAF açısından daha saf olduğu görülmüştür. Elde edilen tüm katı fazlar kristal MAF olarak tanımlanmıştır.   Anahtar Kelimeler: İdrar, NH3-N geri kazanımı, Magnezyum amonyum fosfat hekzahidrat.In recent years, utilization of source-separated human urine in agricultural farmlands instead of artificial mineral fertilizers has become a favourite method. Urine contributes about 80% of nitrogen and 60% of phosphorus to household wastewater, although it occupies less than 1% of the wastewater volume. Thus, separating urine from household wastewater and recovering the urinary nutrients can be considered as the most important step towards sustainable water and agriculture concepts. Urine fraction utilized in agriculture for vegetation must be free of faeces. To ensure this criterion, urine and faeces are separated at specially designed toilets. Separation of urine in toilets is also known as source separation. Source separating toilets have two bowls where urine and a small amount of flush water are collected in front part of the bowl and the faeces in the back. After the separation urine and mixture passes flush water passes through a separate pipe system to a holding tank. Urine is collected by trucks and transported to storage tanks. When fertilizing season comes, urine is again transported by trucks and applied directly to farmland after being diluted with water. Source separation and separate collection of human urine is mostly preferred in rural area, because small and simpler treatment units are used. Besides, nutrient discharges are minimized (eutrophication is reduced) and nutrient cycle is almost closed. Water is saved and fresh water which is used for flushing is reduced. Finally, the amount of artificial fertilizers which are used for vegetation is lowered. Although source separation is beneficial, there are some disadvantages and risks regarding with the storage, transportation and application of human urine. Researches which have been performed up to now showed that transforming the urinary nutrients into solid state by certain processes minimize the drawbacks. If urine is processed to obtain a urine based solid fertilizer, urinary ammonia which is unstable in the holding tank is stabilized. The amount of urine which is needed to fertilize a farmland is reduced. Ammonia which loss during application is high if urine is applied to the farmland directly. This loss is much more lower when urine based solid fertilizer is preferred. Besides, the amount of urine which is stored and transported decreases. Due to high pH and high NH3-N concentration of stored urine, magnesium ammonium phosphate hexahydrate (MAP) precipitation method is selected as the most appropriate processing method for stabilizing the urinary ammonia. MAP is a white crystalline compound having magnesium, nitrogen and phosphorus in equal molar concentrations. Apart from being a well known kidney stone, this compound is also known to form scales in wastewater treatment plants. MAP is a sparingly soluble salt and solubility decreases with increasing pH. Thus, its low solubility in water enables it to be used as a slow release fertilizer. The present study involves the investigation of the unstable ammonia behaviour during the storage period and the precipitation of MAP from 4 times diluted and stored urine. As a first  step, the change in NH3-N concentration during the storage period was recorded. After determining the final NH3-N, PO43--P and Mg concentrations, MAP precipitation was achieved and optimum process conditions were identified. Each precipitate was identified by X-ray diffraction. FT-IR instrument was used to define the functional groups in each precipitate.According to the results, 6 weeks stored urine contained 0.08 mol/l NH3-N. As the storage period was extended to 14.5 weeks, the NH3-N concentration reached up to 0.14 mol/l. During this period, pH of urine increased with time and the color of urine got darker. As a result of the pH increase, spontaneous precipitation of MAP and hydroxyapatite (Ca5(PO4)3OH) was observed. 86.7% of urinary NH3-N was reacted in precipitation reactions where the pH was adjusted as 9.5 and 10 separately. The percentage recovery of NH3-N decreased to 85.8%  at pH 10.7 which was known as the pH of minimum MAP solubility. In contrast, the percentage recovery of PO43--P and Mg were constant in this pH range. Initial molar concentration ratio of NH3-N, PO43--P and Mg affected precipitation efficiency. According to the results, the percentage recovery of NH3-N and PO43--P increased when the initial molar concentration of Mg was in excess. All the solid phases precipitated from the urine were positively identified as crystalline MAP. Furthermore, all of the  precipitates contained PO43-, NH4, O-H and N-H groups which were all detected in FT-IR analyses.   Keywords: Urine, NH3-N recovery, Magnesium ammonium phosphate hexahydrat