Treatment of olive mill effluents Part II. Complete removal of solids by direct flocculation with poly-electrolytes

The pre-treatment of three different olive oil processing effluents by means of direct flocculation (i.e. without prior coagulation) was investigated. Four cationic and two anionic poly-electrolytes were tested and most of them were found capable of removing nearly completely total suspended solids (TSS) as well as reducing considerably the concentration of chemical (COD) and biochemical oxygen demand (BOD(5)) without altering solution pH. Flocculant dosage was crucial to achieve effective separation. For three cationic and one anionic poly-electrolytes, the minimum dosage required to initiate separation was about 2.5-3 g/L. The remaining two poly-electrolytes failed to cause separation even at dosages as high as 7 g/L. Lime and ferric chloride were also tested as reference coagulants and found quite effective in terms of TSS removal although the degree of COD reduction was generally lower than that with poly-electrolytes. However, lime treatment would require greater dosages and longer treatment times than that with poly-electrolytes and would also increase considerably solution pH. A preliminary cost analysis showed that lime treatment for complete solids removal was generally less costly than that with poly-electrolytes presumably due to its low market price. Nonetheless, cost-benefits may be defied by several drawbacks associated with the use of lime.