Economical and reliable mercury reduction in refinery and power plant discharge wastewater with robust microfiltration membrane technology

Environmental concerns and risks to bioaccumulation of mercury components among aquatic organisms, thus finally through the food-chain, and therefore exposing human health potentially, lead to more stringent regulations over the last years in several regions across the globe. Mercury (Hg) is a key contaminant in several crude oils and coal. Most of the mercury appears to exist in suspended and adsorbed forms and there is also the dissolved component in the form of methylmercury and salts. As an example, the United States EPA’s Great Lakes region initiative targets limiting the total mercury in lakes and streams to 1.3 nanogram/l (ppt). Many states in the USA along the Great Lakes and the Ohio valley limit total mercury in discharge wastewater to 12 nanogram/l (ppt) already. Western Europe and parts of Asia are most likely to implement similar mandates in the next few years. This paper describes in the first case a lengthy and successful plant trial performed by Pall Corporation on discharge wastewater generated at a refinery in the USA. Microfiltration (MF) systems were employed to treat the end of pipe wastewater after it goes through conventional treatment processes such as API separator and biological reactor treatment. The total mercury was reduced to below 5 ppt in the permeate from the MF process without the aid of additives or post treatment, thus reducing 90% of the total mercury in the feed to the MF system. The target levels were met consistently and the automated system ran smoothly. Independent of huge variability of the feed turbidity and changing mercury levels, the hollow-fiber system showed stable and reliable operation for more than 12 months under “real” site conditions. Above innovative approach eliminates the need for further mercury polishing steps while reaching target levels with a very solid and proven high-crystalline PVDF hollow fiber, that collect solids and contaminations only on the outer membrane side, which enable easy regeneration and cleanability all the time. The second case study refers to the power generation industry. Wet flue gas desulfurization (FGD), used primarily to control SOx in coal-fired power plants, also captures the Hg in its various forms. The wastewater generated in this process is contaminated accordingly, thus needing treatment for Hg removal before discharge to the environment. This second case study discusses experimental work done by Pall for the removal of Hg from FGD wastewater as a final polishing step, with and without chemical precipitation followed by MF to remove the Hg captured in the precipitate. Stable and positive results from both case studies could be obtained, hence proving that high crystalline PVDF in hollow fiber membranes is a simple, reliable, long-term and cost economic solution for the reduction of mercury compounds in wastewaters.