Süt endüstrisi atıksularının havasız arıtımı

Özet

Bu makalede, süt endüstrisi atıksularının laboratuvar, pilot ve tam ölçekli havasız arıtımı konusunda son yıllarda yapılan araştırmaların bir özeti sunulmaktadır. Makalede süt endüstrisi atıksularının tek ve iki kademeli havasız arıtımı irdelenerek; aerobik ve anaerobik proseslerin birlikte kullanımı detaylı olarak incelenmiş ve bu konuda daha fazla araştırma yapılması gereken alanlar belirtilmiştir. Süt endüstrisi atıksularının arıtımında, fıziksel-kimyasal ve aerobik metotlara göre daha önemli avantajlara sahip olmaları nedeni ile konvansiyonel (tek kademeli) ve iki kademeli havasız (anaerobik) arıtma prosesleri kullanılmaktadır. Bu prosesler içinde, özellikle yukarı akışlı çamur yatağı havasız reaktörlerden sıkça faydalanılmaktadır, çünkü bu reaktörler kısa zaman içinde yüksek miktarda atıksu arıtabilme kapasitesine sahiptirler. Süt endüstrisi atıksularında mevcut, yüksek miktarda lipid ve askıda katı madde miktarı özellikle tek kademeli havasız arıtma sistemlerinin performanslarını olumsuz biçimde etkilemektedir. Bu yüzden, iki kademeli havasız arıtma prosesleri süt endüstrisi atıksularının arıtımında önemli bir alternatif olarak değerlendirilebilir. Süt endüstrisi atıksularının iki kademeli havasız arıtımı, özellikle asit kademesinin mikrobiyolojisi ve de metan fazının performansı açısından daha detaylı incelenmelidir. Ayrıca havasız arıtma prosesleri metan ve hidrojen gibi yenilenebilir enerji kaynakları üretimi çok önemli potansiyele sahip olduklarından, yüksek organik yüke sahip süt endüstrisi atık sularından biyogaz üretimi de üzerinde durulması gereken bir konudur. İlgili yönetmelikler çerçevesinde belirlenen atıksu deşarj limitlerine ulaşmak için süt endüstrisi atıksularının arıtımında aerobik prosesler de anaerobik prosesler ile beraber sıkça kullanılmaktadır. Özellikle azot ve fosfor giderimi açısından bu proseslerin performansları çok önem taşımaktadır ve bu konuda güncel çeşitli çalışmalar mevcuttur.

Abstract

For treatment of dairy wastewaters, biological conventional single and high-rate two-phase anaerobic treatment processes are commonly used, since they have more important advantages than physical-chemical and other biological methods. In this review article, a summary of recent laboratory, pilot and full-scale research activities on anaerobic treatment of dairy wastewaters are presented. In the article, single and two-phase anaerobic treatment of dairy wastewaters are firstly evaluated; combined use of aerobic and anaerobic processes are investigated in detail and areas where further research required are determined. In single-phase anaerobic treatment of dairy waste-waters, anaerobic filter and upflow anaerobic sludge blanket (UASB) reactors are commonly used. Anaerobic filters usually provide higher chemical oxygen demand (COD) removals around 90 %. Generally, anaerobic filters are operated in an organic loading rate range of between 5 to 10 kg COD/m3/day. However, low suspended solids (SS) and lipid concentrations in dairy effluents are key factors of success in anaerobic filter treatment. UASB reactors are another imperative alternatives for biological treatment of dairy waste effluents, since these reactors can treat larger volumes of wastewater, relatively at lower hydraulic retention times (HRT). Higher organic loading rates (OLR) are therefore applicable. However, biogranulation mechanism is the key parameter in UASB reactor systems, to attain a higher process efficiency and stability, independent of the wastewater type to be treated. UASB reactor systems are able to provide more than 90 % COD removal and can be operated in an OLR range up to 22 kg COD/m3/day. More research is particularly required on the microbiology of UASB reactor systems, to understand the mechanism of granulation clearly, so that the rates, when operated in an OLR range from 6 to 11 y can extensively be used in biological treatment of dairy and other seasonal industrial wastewaters. In addition to anaerobic filter and UASB reactor systems, hybrid reactors and anaerobic sequencing batch reactors (ASBR) are also employed in anaerobic biological treatment of dairy wastewaters. Both processes are able to provide higher than 90 % removal rates, when operated in an OLR range from 6 to 11 kg COD/m3/day. Two-phase anaerobic treatment systems are also important alternatives for anaerobic treatment of dairy wastewaters. During two-phase anaerobic treatment processes, the performance of the first phase -acidogenesis- is of paramount importance. The most appropriate substrate should be provided in the acidogenic phase, to the subsequent methanogenic phase, for successful conversion to methane during methanogenesis. High-rate two-phase anaerobic treatment systems seem quite useful for treating dairy effluents, which contain particularly high concentrations of COD, suspended solids and lipids. For two-phase anaerobic processes, COD removals vary between 90 and 95 %. The flow rate and the nature of the particular dairy waste stream, the concentrations of suspended solids, lipids and proteins, and the variations in production cycles are the most important parameters, which affect the stability and performance of both conventional single-phase and high-rate two-phase anaerobic treatment systems. Two-phase anaerobic treatment processes seem more convenient for dairy streams with a high content of SS and lipids. Besides, there must be a sufficient amount of alkalinity present in anaerobic reactors for a stable operation. Anaerobic treatment systems also generate biogas, a valuable source of renewable and clean energy. Hydrogen and methane are produced via conventional single and two-phase anaerobic treatment of dairy wastewaters, since particular dairy waste streams contain a high organic load. In addition to anaerobic treatment processes outlined above, combined aerobic-anaerobic treatment systems are commonly used, to treat dairy wastewaters. The primary objectives in these treatment systems are to reduce the concentrations of nitrogen (N) and phosphorus (P) released, and to achieve the high effluent discharge limits currently enforced by regulations. Finally, two-phase anaerobic treatment of dairy waste effluents, combined aerobic-anaerobic biological treatment systems, and production of renewable biogas from dairy wastewaters are particular areas, where further and more detailed attention should be directed towards in near future.