Influence of pH on soluble substrate generation with primary sludge fermentation

Çalışma kapsamında pH’nın ön çökeltme çamuru fermentasyonu sonucu çözünmüş besi maddesi oluşumuna etkisi incelenmiştir. Bu kapsamda pH kontrollü ve konrolsüz koşullarda paralel deney setleri yürütülmüştür. 20°C sabit sıcaklıkta gerçekleştirilen ön çökeltme çamuru fermentasyonu sonucunda çözünmüş Kimyasal Oksijen İhtiyacı (KOİ) salımı ortalama 14 mg/L olarak hesaplanmıştır. Bu değere karşı gelen Uçucu Yağ Asiti (UYA) oluşumu ise 11.7 mg/L KOİ olarak hesaplanmıştır. Elde edilen bu değer, toplam biyolojik ayrışabilir KOİ içeriğinde %5 artışa karşı gelmektedir. Bu değere karşı gelen UYA üretimi ise ortalama 9.2 mg KOİ/L olarak belirlenmiştir. Fermentasyon sonrasında azot (0.4 mg/L) ve fosfor (0.1 mg/L) şeklinde besi maddesi salımlarının ihmal edilebilecek düzeyde olduğu gözlemlenmiştir. Yapılan bütün deneylerin sonucunda asetik asit üretimi toplam UYA’nın %45’inden fazlasını oluşturmuştur. Yüksek pH değerlerinde ve başlangıç UAKM değerlerinin arttığı durumlarda UYA’nın asetik asit içeriğinde düşüş, propiyonik asit ve C4-C5 asitlerinin oranlarında artış gözlenmiştir. Ön çökeltme çamurunda pH ayarının ve kontrolünün 5.5 ve 6 değerlerinde fermentasyon reaksiyonunu olumsuz yönde etkileyerek daha düşük konsantrasyonda UYA oluşumuna neden olduğu belirlenmiştir. Aynı zamanda pH ayarının ve kontrolünün 5.5 ve 6 değerlerinde asidojenik fazda gecikmeye ve reaksiyon süresinin uzamasına neden olduğu görülmüştür. Yüksek pH değerlerinde ise (pH 7.5) hidroliz fazında iyileşme saptanmış, ancak düşük pH değerlerinde olduğu gibi yüksek pH değerlerinin de asidojenik fazı olumsuz etkilediği gözlemlenmiştir. Çamurun başlangıç pH değerine yakın olan pH 6.5 ve 7 kontrollü setleri ise kontrolsüz setlerle aynı sonuçları verdiği için pH kontrolünün ekonomik anlamda uygulanabilir olmadığı belirlenmiştir. Anahtar Kelimeler: Besi maddesi giderimi, çözünmüş fermentasyon ürünleri, KOİ bileşenleri, kütle dengesi, uçucu yağ asitleri.A significant function of primary sludge fermentation is now regarded as the generation of soluble organic compounds, in particular Volatile Fatty Acids (VFAs), that are necessary for Biological Nutrient Removal (BNR) processes. VFAs are by nature readily biodegradable and therefore most suitable carbon source for denitrification. Their presence in sufficient quantity is also required for biological phosphorus removal systems. The merit of primary sludge fermentation is partial conversion of the settled Chemical Oxygen Demand (COD) into soluble and mostly readily biodegradable components and especially VFAs. In fact, pre-fermentation essentially sustains the first phase of anaerobic biodegradation. It is basically characterized by the generation of soluble COD. The study was undertaken to evaluate the effect of pH control on the generation of soluble fermentation products from primary sludge. The effect was tested by running parallel experiments under pH controlled and uncontrolled conditions. Primary sludge samples were taken from the Atakoy treatment plant, a small domestic wastewater treatment facility located in Istanbul, Turkey. The effect of pH on primary sludge fermentation was assessed using a series of experiments where the pH of the anaerobic reactor was sequentially adjusted and maintained at the following pH values: 5.5, 6, 6.5, 7, and 7.5 throughout the tests (controlled series). In fermentation experiments conducted at 20 ºC without pH control, the average soluble COD release was 14 mg per liter of wastewater treated, representing a potential increase of 5% in the biodegradable COD content of the primary sedimentation effluent. 27% lower VFA production was observed at pH 5.5, 18% at pH 6.0 and 12% at pH 7.5. The corresponding average VFA generation was 9.2 mg COD/L. Similarly, soluble COD generation was 10% lower at pH 5.5 and remained practically the same at pH 6.0. Increasing the pH to 7.5 resulted in 26% higher soluble COD generation indicating that higher pH favored hydrolysis but not acidification. Experimental assessment of VFAs composition indicated the predominance of acetic acid in fermentation products. In the majority of runs, the acetic acid fraction of the total VFA was over 45%. The average COD equivalent of VFAs was computed as 1.36 mg COD mg/L VFA for uncontrolled pH conditions. Increasing the pH level of primary sludge fermentation, the acetic acid percentage of the generated VFAs significantly decreased leading to the production of more propionic acid and C4?C5 (butyric, izobutyric, valeric ve izovaleric) acids. The highest percentage of the acetic acid production was around 70%, observed at pH 5.5. In experiments where pH was not controlled, the soluble TKN concentration released changed in the range of 75?175 mg/L with an average value of 97 mg/L, presumably depending on wastewater quality and resulting primary sludge characteristics. The soluble TKN represented on the average 2% of the VSS reduced, 4.6% of the soluble COD released and 5.7% of the VFA COD generated (0.057 mg N/mg CODVFA). Soluble P release was practically the same for experiments with pH controlled and uncontrolled runs with an average value of around 20 mg/L. This level corresponded to 1.3?1.4% of the VFA COD produced (0.013 mg P/mg CODVFA). Sludge fermentation experiments without pH control induced an average soluble COD release of 14 mg/L, which varied in the range of 8.4?19.6 mg COD/L. In terms of mass balance, this represents 5% of the biodegradable COD in the primary sedimentation effluent, which would be increased to 279 mg COD/L if the fermenter supernatant were to be added back to the effluent stream. The corresponding average VFA generation was calculated as 11.7 mg COD/L. Also, 0.6 mg/L of the nitrogen and 0.1 mg/L of phosphorus removed by settling were released with primary sludge fermentation. pH adjustment and control outside the initial pH of the primary sludge had a negative effect on VFA generation. Two general comments may be associated with the pH control (i) delay in acidification/ longer fermentation times (ii) lower VFA production. pH adjustment and control in the range of 5.5?6.0 had a negative effect on fermentation efficiency, mainly observed as lower VFA generation and delay in acidification and longer fermentation times. A similar negative effect was also observed when the pH was increased to 7.5. pH control in the 6.5?7.0 range, close to the initial pH of the primary sludge essentially yielded the same results and did not prove meaningful. Keywords: Nutrient removal, soluble fermentation products, COD fractions, mass balance, volatile fatty acids