Application of microfiltration process to the treatment of olive oil mill wastewaters

Mikrofiltrasyon işleminin zeytinyağı endüstrisi atıksularına uygulanabilirliği

Abstract

Olive oil mill wastewater (OMWW) is an important environmental pollution problem in Mediterranean countries. Although the duration of campaign for processing olives is continued from November to February, the amount and pollution potential of wastewater are very high. Characteristics of olive mill wastewater depends on the extraction process used and the operating conditions. The main organic constituents of olive mill wastewater are sugars, nitrogenous compounds, volatile acids, polyalcohols, proteins, fats and polyphenol. The main biological and physicochemical characteristics of the OMWW are as follows: BOD: 15000-135000 mg/L, COD: 37000-318000 mg/L, SS: 6000-69000 mg/L, pH: 4.6-5.8 (Oktav and Ozer, 2004). The disposal of highly pollutant olive by-products, especially the aqueous liquor, is an important environmental problem, which needs to be solved. Flexible and efficient treatment plants are needed for the treatment of OMWW; these should assure not only a significant reduction of BOD and COD values, but also the possibility of selectively recovering some valuable compounds that could be used in the same production cycle or as raw material for other process. For these aims, membrane processes should be applied (Turano and others, 2002). One common problem of membrane filtration of OMWW is the membrane fouling that drastically reduces the efficiency of permeation and also changes its selectivity. Therefore, a pre-treatment step is necessary to decrease membrane fouling and to increase filtration efficiency. By considering this fact, the study was designed to evaluate the effect of different pretreatment methods on performance of membrane process in OMWW treatment. Physical and chemical pretreatment steps were applied before microfiltration in a flat-sheet membrane module, separately. Cartridge filter filtration and filter cloth experiments were done as physical pretreatment studies. OMWW was first filtered from a 20 m cartridge filter which was inserted into the influent line to protect the membranes from suspended solids. Wastewater flow rate was 0.4 L/s. Wastewater then filtered from four different filter cloths with different pore size, in membrane cell. All filter cloths were made up of polyester fabric. Filtration experiments were done at 100 L/h flow rate and the concentrate flow control valve (CFCV) was kept open. Filter cloths were used in order from more permeable to less permeable. The main characteristics of OMW after physical pretreatment were: COD: 77700 mg/L; SS: 3530 mg/L; oil and grease: 1110 mg/L. As chemical pretreatment experiments, pH of wastewater was first adjusted to pH=2, and then to pH=7. About 50% COD and SS removal were achieved by pH adjustment. However application of cartridge filter and filter cloths to chemical pretreatment effluent increased the efficiency to 60% for COD and 78% for SS. In the microfiltration experiments, the effect of flow rate and pressure on flux and concentration of organic substance were investigated. The flow rate was varied between 100 and 200 L/h and pressure was controlled at 1 and 2 bars. One set of experimental set was carried out by keeping the concentrate flow control valve (CFCV) open. Permeate flux became independent of the time for all flow rate and pressure couples. So, all flux experiments were finished after 120 minutes. Pressure and recycle flow rate significantly influenced the permeate flux. Higher flow rate at the membrane surface is a very important factor in increasing the permeate flux. Using higher velocity, the deposited particles are continuously removed from the membrane surface and thus the hydraulic resistance of the fouling layer is reduced. Increase in pressure also increased water fluxes for all pretreatment options. Maximum water fluxes were obtained at 200 L/h flow rate and 2 bar pressure. It can be clearly seen that, fluxes are bigger for the chemically pretreated wastewater. Probably, the flock formation on membrane surface during the microfiltration might increase the flux like filter press filtration. If the pressure of system was increased, COD and TOC concentration of samples also increased. In addition, increasing flow rate also resulted an increasing in COD and TOC concentration. Maximum removal efficiencies were achieved at 100 L/h flow rate and open CFCV condition. The experimental results indicated that higher water fluxes and removal efficiencies can be obtained by chemical pretreated compared to physical treatment methods. Removal efficiencies were 98.0%, 94.0%, 75.4%, and 74.2% for SS, oil-grease, TOC, and COD parameters, respectively, with the combination of chemical pretreatment and membrane filtration process. However, the effluent water quality after applied treatment technologies does not satisfy the discharge standards in Turkey. Therefore, the performance of the other advance treatment technologies should be investigated.

Özet

Bu çalışma kapsamında, 3 fazlı zeytinyağı üretimi yapan bir tesisten alınan karasu numunesi ile çalışılmış, bu numunenin fiziksel ve kimyasal ön arıtımından sonra mikrofiltrasyon prosesi ile arıtılabilirliği incelenmiştir. Karasuyun fiziksel ön arıtımı amacıyla, kartuş filtreden ve piyasada hava geçirgenliğine göre satılan dört farklı filtre bezinden filtrasyon denemeleri yapılmıştır. Böylece ham numunenin 120000 mg/L olan KOİ konsantrasyonu 77700 mg/L’ye düşürülmüştür. Kimyasal ön arıtımda ise iki kademeli koagülasyon denenmiştir. Numunenin pH’ı önce 2’ye, sonrasında ise 4’e ayarlanmış ve daha sonra kartuş filtreyi takiben filtre bezlerinden süzülmüş, böylece 48000 mg/L KOİ konsantrasyonuna ulaşılmıştır. Fiziksel ve kimyasal ön arıtımdan geçirilmiş olan karasu numuneleri ayrı ayrı mikrofiltrasyon işlemine tabi tutulmuştur. Mikrofiltrasyon denemeleri, 100 – 150 ve 200 L/saat debi ve atmosferik basınç, 1 ve 2 bar basınç altında yapılmıştır. Debi ve basınçtaki artış süzüntü akısını ve KOİ ile TOK konsantrasyonlarını arttırmıştır. Kimyasal olarak ön arıtılmış numuneyle daha yüksek akı değerlerinin elde edildiği görülmüştür. Kimyasal arıtma sonrasında uygulanan mikrofiltrasyon işlemi ile elde edilen giderme verimlerinin, fiziksel ön arıtılmış numunenin mikrofiltrasyonu ile elde edilen sonuçlarına göre daha yüksek değerlerde olduğu belirlenmiştir. Kimyasal arıtma ve mikrofiltrasyon kombinasyonu sonucunda en yüksek giderme verimi (%98) AKM parametresi için elde edilirken, yağ-gres için %94, TOK için %75.4, KOİ için ise %74.2 giderme verimleri gözlenmiştir. Buna rağmen ulaşılan konsantrasyonlar hala yüksek mertebededir ve ilave arıtma teknolojilerine ihtiyaç duyulmaktadır.