Journals / İTÜ Dergisi Seri E: Su Kirlenmesi Kontrolü / 2007 / Cilt: 17 - Sayı: 2
Effect of primary sludge fermentation products on mass balance for biological nutrient removal system
- Pages
- 37–47
- DOI
- —
Abstract
Nutrient removal from wastewater is the main concern, especially in areas sensitive to eutrophication. Coastal zones open to touristic activities require wastewater treatment involving maximum nutrient control in order to maintain the delicate balance between excessive land use and desired water quality. Effective biological nitrogen and phosphorus removal requires full utilization of the internal pool coming from the process influent. In most cases, primary settling is the commonly prescribed step before biological wastewater treatment. It removes a significant portion of the influent COD, around 30 to 40% in domestic sewage. Primary settling step is sometimes omitted to increase the internal carbon potential in cases where the magnitude of the COD available in the influent is critically limiting for the desired level of nutrient removal. It is a fact that, the nature and the biodegradation rate of the available carbon are important factors for an efficient system operation. The COD fractionation identifying fractions with different biodegradation rates has been a milestone in understanding and modeling the substrate utilization, especially in nutrient removal. This study evaluates mass balance applicable to internal organic carbon pool of domestic sewage in terms of different COD fractions, with the specific objective of investigating the potential of simple (uncontrolled) primary sludge fermentation for the generation of readily biodegradable substrate. Fermentation products were also evaluated by means of respirometric analyses. Batch experiments were conducted, at 20°C constant temperature, to investigate the potential of primary sludge fermentation for the generation of readily biodegradable substrate and to evaluate the effect of primary sludge fermentation products on mass balance including nitrogen and phosphorus. Limited fermentation without pH control converted 17% of the particulate COD removed from sewage by means of primary settling to simpler soluble compounds, mainly to VFAs through acidification of the sludge in the fermenter. Experimental evaluation shows that fermentation converted between 18 to 30% of the initial volatile suspended solids (VSS) in the sludge into soluble biodegradable COD. Approximately 85% of the soluble COD is volatile fatty acids after the fermentation process. The net yield giving the fraction of the total COD in the primary sludge converted into volatile fatty acids (VFA’s) varied in the range of 0.095 to 0.19 g VFA COD/g COD. The average fraction of the VFA’s in fermentation is 47% acetic acid, 35% propionic acid, 9% butyric acid and 9% valeric acid. Based on these observations, the COD equivalent of VFA was calculated as 1.38 mg COD/mg VFA. The recoverable fraction of the fermented sludge supernatant was calculated to have the potential of increasing the biodegradable COD content of the primary effluent by 5% and the readily biodegradable COD in the effluent by 30%. The results clearly showed that the predominant fermentation products are always acetic and propionic acid, independent of the values of the operational parameters. The most important volatile fatty acids obtained during the biological fermentation process is acetate with approximately half of total VFA concentration, which is one of the most important carbon source for denitrification and biological nutrient removal processes. Ammonium and phosphate release during biological fermentation process were less than 1% and 2%, respectively. Therefore, the fermentation may be used directly in the BNR process without any separation of ammonia and phosphate. Primary settling reduced NDP of the raw sewage by 17 to 29%, approaching the level of the COD fraction removed as VDN/V ratio increases. Fermentation products provided an additional denitrification potential of around 2 mg N/L, increasing NDP of the primary effluent in the range of 10 to 20% depending on the gradual decrease of the operating VDN/V ratio from 0.4 to 0.1. Model simulation indicated that fermentation products could improve the biological P removal potential of the primary effluent reducing the effluent P concentration from 2.7 mg P/L to 0.6 mg P/L. The recoverable fraction of the fermented sludge supernatant may potentially increase the readily biodegradable COD content of the primary effluent by 5% and soluble nitrogen and phosphorus content by 2%.
Özet
Ön çökeltme çamuru fermentasyonunun kolay ayrışabilir organik madde oluşturma potansiyeli ve karbon, azot ve fosfor olmak üzere oluşan ön çökeltme ürünlerinin kütle dengesine olan etkisi, laboratuvar ortamında, 20°C sabit sıcaklıkta gerçekleştirilen kesikli deneyler ile incelenmiştir. Fermentasyon sonucunda, ön çökeltme çamurundaki uçucu askıda katı madde’nin %18’den %30’a varan oranlarda çözünmüş biyolojik ayrışabilir KOİ’ye dönüştürüldüğü gözlenmiştir. Fermentasyon sonrasında, çözünmüş KOİ’nin %85’inin uçucu yağ asitlerinden oluştuğu belirlenmiştir. Uçucu Yağ Asitlerinin (UYA) dağılımı, %47 asetik asit, %35 propiyonik asit, %9 bütirik asit ve %9 valerik asit olarak bulunmuştur. Toplamda oluşan UYA’ların KOİ karşılığı 1.38 mg KOİ/ mg UYA olarak hesaplanmıştır. Biyolojik fermentasyon sonrasında elde edilen ve toplam uçucu yağ asitlerinin yaklaşık yarısını oluşturan asetat, denitirikasyon ve biyolojik besi maddesi gideren sistemlerde çok önemli bir karbon kaynağıdır. Ön çökeltme çamurunun fermentasyonu sonucunda, çözünmüş azot üretiminde 0.7 - 3.6 mg NH4-N/g KOİ ve çözünmüş fosfor üretiminde 0.3 - 0.8 mg PO4-P/g KOİ dönüşüm oranları belirlenmiştir. Fermentasyon ürünlerinin, 0.4’ten 0.1’e doğru düşen anoksik hacmin proses hacmine oranı (VDN/V) çalıştırma koşullarına bağlı olarak ön çökeltme çıkış akımında denitrifikasyon potansiyelini (NDP) %10 - 20 arasında arttırdığı görülmüştür. Fermente olmuş ön çökeltme çamurunun üst fazının geri kazanılabilen kısmı, çıkış akımının kolay ayrışabilir KOİ içeriğini %5 ve aynı zamanda çözünmüş azot ve fosfor içeriğini de %2 arttırabilmektedir.