Evaporation and air-stripping to assess and reduce ethanolamines toxicity in oily wastewater
Introduction
There has been a consistent rise in xenobiotic compounds release into the aquatic environment in recent decades due to the rapid increase in wastewater industrial production. Most of these substances are suspected to be toxic and carcinogenic, and generally have low biodegradability. This fact, combined with the high organic pollution load in terms of chemical oxygen demand (COD) and matrix effect [1], makes the purification of such wastewater a difficult task [2], [3], [4]. In Italy, as in many other Mediterranean countries, oily wastewater is a major environmental problem in the coastal zone, where the chemical industry produces significant amounts of industrial waste [5].
Control of toxic pollutants is extremely troublesome, time consuming and costly, especially in industrial areas where a large number of complex effluents are collected, combined and treated in wastewater treatment plants (WWTPs) [6]. Kahru et al. [7] proposed a battery of microbiotests for evaluating wastewater pollution from the oil shale industry, showing that chemical analyses can easily miss contaminants with a high toxic impact. Performing a hazard assessment with an appropriate battery of toxicity tests can integrate traditional characterisations for assessing impacts on the target environment. Oily wastewater generated by various industries, frequently occurring in the form of oil-in-water emulsion, creates a major problem around the world [2], [8], [9]. Oily wastewater is generated by different activities such as refinery, petrochemical and lubricant production units, metal finishing, metal working, textile industry and paper mills [4], [10] and can have a complex composition because it may contain mineral, vegetal or synthetic oils, fatty acids, emulsifiers, corrosion inhibitors and bactericides.
This study tested a method to reduce the impact on the aquatic environment of oily wastewater of industrial origin, containing several organic pollutants with varying biodegradability characteristics. Oily wastewater emulsion, after the first step of breaking by a chemical method, was treated by a sequence of evaporation and air-stripping physical processes, in order to verify the removal efficiency of organic and inorganic pollutants as well as the toxicity reduction performance. Evaporation and air-stripping were tested in a lab-scale plant facility, while a full-scale WWTP was used as a case study for the evaporation process. Traditional physical and chemical characterizations for raw and treated wastewaters were integrated with Microtox® bioassay results. Furthermore, according to the toxicity reduction evaluation procedure (TRE) [11], a biomonitoring screening survey with Vibrio fischeri involved the control of monoethanolamine (MEA, CAS Number: 75-04-7), diethanolamine (DEA, CAS Number: 111-42-2) and triethanolamine (TEA, CAS Number: 102-71-6) roles in toxicity definition, due to their frequent occurrence as emulsifiers and corrosion inhibitors in oily wastewater. They were all monitored as pure substances and as mixtures (MEA and TEA) in order to implement part of a general project of toxicity reduction assessment.
Section snippets
The TRE approach
A TRE investigation is designed to isolate the source of effluent toxicity and determine the effectiveness of various control options (e.g. technological facility) in reducing toxicity. In this work, the TRE procedure was applied to oily wastewater treatment in both a lab-scale plant and a full-scale WWTP. This approach was similar to the USEPA [11] general approach which involves three tiers. The first tier involves the collection of background information on the plant and its past operating
Materials and methods
The lab-scale plant consisted of a vacuum evaporation (roto-vapour) device, which can operate at different temperatures, and an air-stripping apparatus. The evaporating apparatus was composed of a 1 L Pyrex flask in a thermostatic bath, thermometer, cooler, 100 mL graduated cylinder for condensate collection, an oil rotative vacuum pump and pressure gauge. The air-stripping tests were conducted in a 100 mL beaker with an air bubbling device (50–70 L/h, T = 50 °C and P = 1 atm) in a thermostatic bath, in
Physical and chemical parameters evaluation
The evaporation process in the lab-scale plant considered an oily wastewater characterised by COD = 14,760 mg O2/L, N–NH4 = 1190 mg/L, pH 8.2 and TU50 = 70.00. MEA, DEA and TEA concentrations were not determined in the oily wastewater for the lab-scale plant, because their presence was not at first suspected.
The results for evaporated and condensated samples (F1–F5) are shown in Fig. 3 and summarized as removal efficiencies in Table 3, Table 4. In the condensate samples, pH values remained constant
Conclusions
This study investigated a procedure for the treatment of oily wastewater posing a serious problem for the aquatic environment. The procedure consisting of a sequence of evaporation and air-stripping processes was tested in a lab-scale plant. The evaporation procedure was also validated in a full-scale WWTP. COD, ammonia and TU50 (via Microtox®) were monitored. After a chemical breaking procedure, oily wastewater was evaporated at three different temperatures (T = 50, 70 and 80 °C) and the
Acknowledgment
Alison Garside revised the English text.
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