Assesment of chemical treatability of olive mill wastewaters by coagulation and electrocoagulation

Bir zeytinyağı karasuyunun koagülasyon ve elektrokoagülasyon prosesleriyle kimyasal arıtılabilirliğinin incelenmesi

Abstract

Olive oil industry is of vital economic importance for many Mediterranean countries, accounting for approximately 95% of the olive oil production worldwide. Generation and disposal of around 30 million m3 of olive mill wastewater per year constitute a serious environmental problem for the Mediterranean Region, due to the unique features associated with this type of agro-wastewater, namely seasonal (typically between November and February) and localized production and the high organic carbon content quite resistant to biodegradation. The characteristics of olive mill wastewater generated in the small-to-medium scale companies largely depend on the type of extraction process employed -the traditional press method or the continuous, threephase centrifugation process. Accordingly, the high and refractory organic content of olive mill wastewater varies in the range of 37 – 318 g/L chemical oxygen demand (COD), 15 – 135 g/L biochemical oxygen demand (BOD5), 6-69 g/L suspended solids and 2.5 g/L total phenolic content (TPh). Different physical, chemical and biological processes have so far been proposed for olive mill wastewater treatment. Recently, olive mill wastewater was subjected to sequential coagulation/ flocculation/ sedimentation /filtration processes as well as lime treatment, resulting in 62–73% phenolics removal depending upon the process applied for olive oil extraction. More than 40% COD and about 95% oil and grease removals were obtained in these studies. Olive mill wastewater could also be partially treated via coagulation using different polyelectrolyte materials; nearly complete reduction in suspended solids was accomplished accompanied with partial reductions in COD and BOD5 up to 55% and 23%, respectively. The use of conventional coagulants such as alum and ferric chloride was also reported, resulting in 90-91% total phenols and 94- 95% COD removals after preliminary acid cracking. Biological treatment methods such as the activated sludge process and anaerobic digestion were also investigated, all reporting different operating problems due to the high toxicity and bio-inhibitory effect of the olive mill wastewater on heterotrophic biomass. Therefore, the elimination of polyphenolic compounds from olive mill wastewater via appropriate chemical pretreatment methods was considered as an important criterion for toxicity reduction. As briefly summarized above, results of previous experimental work on the treatability of olive mill wastewater, although extensive, are too diverse and conclusive, mainly because they only focus on collective parameters describing the organic carbon content of wastewater, such as COD, BOD5, etc. Within the scope of the present experimental study, organic carbon removal from olive mill wastewater via different chemical treatment processes (coagulation and electrocoagulation) was investigated. For this purpose, hydrated lime (Ca(OH)2), ferric chloride (FeCl3.6H2O), alum (Al2(SO4)3.18H2O) and commercial grade iron base coagulant and coagulant aid were employed as coagulants. Highest COD and TOC removals were obtained with lime precipitation at pH 11 as 49% and 38%, with 1000 mg/L ferric chloride at pH 7.0 as 44% and 53%, with 1500 mg/L alum at pH 6.5 as 40% and 36%, and with commercial grade iron base coagulant and coagulant aid as 46% and 43%, respectively. Highest organic carbon removal was achieved with electrocoagulation using stainless steel electrodes resulting in 60% and 65% COD and TOC abatement respectively, accompanied with 55% antioxidant activity and 52% total phenolics removals. It could be concluded that applying electrocoagulation (with stainless steel electrodes) that has recently received great interest because of its enigmatic treatment performance, resulted in considerably higher treatment performance as compared to the other chemical treatment processes due to its combinative abatement modes featuring redox reactions as well as coagulation, adsorption, precipitation and flotation. Considering the obtained experimental results, it can be concluded that coagulation and electrocoagulation are suitable treatment options for organic carbon and phenolic matter removal from olive mill wastewater. In the light of the experimental findings, it can be inferred that all investigated chemical processes are suitable for chemical pretreatment of olive mill wastewater.

Özet

Zeytinyağı üretiminde ortaya çıkan karasuyun arıtımı yüksek biyokimyasal oksijen ihtiyacı (BOİ5 = 15-135 g/L), kimyasal oksijen ihtiyacı (KOİ = 37-318 g/L), askıda katı madde (AKM = 6-69 g/L) ve toplam fenol (TF = 2-5 g/L) içeriği nedeniyle önem taşımaktadır. Zeytinyağı endüstrisi atıksuları aynı zamanda 10 g/L’yi aşan fenolik ve tannik asit içerikleri nedeniyle biyolojik olarak zor ayrışan bir yapıya sahip olup antimikrobiyal aktiviteye, başka bir deyişle toksisiteye sebep olmaktadırlar. Bu çalışmada, zeytinyağı karasuyundan koagülasyon ve elektrokoagülasyon yöntemleriyle organik madde giderimi hedeflenmiştir. Sönmüş kireç (Ca(OH)2), demir(III) klorür (FeCl3.6H2O), alum (Al2(SO4)3.18H2O) ve demir bazlı ticari koagülan ve pıhtılaştırma yardımcısı kullanılarak gerçekleştirilen koagülasyon deneylerinde elde edilen en yüksek KOİ ve TOK giderim verimleri, kireç ile çöktürme için pH 11’de % 49 ve % 38, demir(III) klorür ile çöktürme için pH 7.0’de 1000 mg/L FeCl3 dozajında % 44 ve % 53, alum ile çöktürme için pH 6.5’da 1500 mg/L dozajda % 40 ve % 36 ve demir bazlı ticari bir koagülan ve pıhtılaştırma yardımcısı ile çöktürme için ise % 46 ve % 43 olarak belirlenmiştir. Uygulanan kimyasal arıtma prosesleri arasında en yüksek organik madde giderimi, çelik elektrodlar kullanılarak gerçekleştirilen elektrokoagülasyonda KOİ ve TOK bazında sırasıyla % 60 ve % 65 oranlarında elde edilmiştir. Elde edilen deneysel sonuçlar değerlendirildiğinde, koagülasyon ve elektrokoagülasyon arıtma proseslerinin organik karbon ve fenolik madde gideriminde yaklaşık olarak aynı düzeyde giderim sağladığı sonucuna varılmıştır. Bu çalışma çerçevesinde incelenen kimyasal arıtma proseslerinin karasu ön arıtımı için uygun ve uygulanabilir yöntemler olduğu tespit edilmiştir.