The use of Advanced Oxidation Processes (AOP) for the treatment of landfill leachate

Inżynieria i Ochrona Środowiska 2012, t. 15, nr 2, s. 203-217 Paulina JELONEK, Ewa NECZAJ Czestochowa University of Technology, Faculty of Environmental Engineering and Biotechnology Brzeznicka 60a, 42-200 Częstochowa The use of Advanced Oxidation Processes (AOP) for the treatment of landfill leachate Formation of leachate poses a problem closely related to the use of landfill sites. Landfill leachate is a wastewater, which as a result of permeation elutes mineral and organic compounds from a bed. Due to its diverse composition, both physical and chemical, it is necessary to purify the leachate before its discharge into drains or a natural receiver. The following article shows that there is a variety of pollutants in municipal landfill leachate. It also presents the dependency existing between the age of a landfill site, and the concentration of pollutants in leachate, as well as how it affects biodegradation of contaminants. There were discussed various methods to purify leachate and the physical, chemical, physico-chemical and biochemical processes were compared. Comparing these processes, many factors were taken into account: the efficiency and effectiveness of the processes, their side effects, the costs, the amount of energy that is required for a process to take place, reaction dynamics, and more. A successful purification of leachate with use of a single process unit is not possible. Therefore, there are used hybrid systems that combine biological methods of physico-chemical processes, especially the advanced oxidation processes. The most important among the chemical processes used for treatment of leachate are Advanced Oxidation Processes - AOP. Advanced oxidation processes are divided into chemical and photochemical oxidation. The most commonly used methods include oxidation with ozone and hydrogen peroxide and Fenton’s reagent oxidation. Processes using ozone are usually employed as the third stage of landfill leachate treatment. This process allows discharge of the leachate into a receiver. The largest decreases in the number of pollutants are obtained using combined processes. Some of the best effects were observed after employment of oxidation both with ozone and hydrogen peroxide, helping to additionally enhance this process photocatalytically. AOP proved to be the most effective method of treatment of waste water that contains organic products (waste water from chemical and agrochemical industries, textiles, paints, dyes). More conventional techniques cannot be used to treat such compounds because of their high chemical stability and low biodegradability. There was also performed a literature review in the field of biological, physical and chemical purification methods of landfill leachate. Keywords: landfill leachate, Advanced Oxidation Processes (AOP), biodegradability enhance- ment Introduction Dynamic development in the industry, as observed over the last year, is causing a growing number of municipal waste and industrial pollution. Landfill is due to economic reasons the most common method of disposal. Exploitation of landfill sites is closely linked with the problem of leachate and the necessity of its purifying. 204 P. Jelonek, E. Neczaj Landfill leachate is a waste water that penetrates a bed and elutes its organic and mineral impurities. The qualitative composition of the leachate determines the biochemical processes that occur during the landfill. Over two hundred substances which are organic leachate pollutants were characterized. Thirty-five of them belong to the so-called “priority pollutants”. With the passage of time in a landfill leachate the concentration of easily degradable organic volatile acids, and other small molecules decreases, while the number of difficult biodegradable compounds - fulvic acid and humic increases [1]. The sources of leachate are: precipitation, inflow of groundwater and surface water, storage of wet material, water losses due to evaporation, transpiration, surface flow, as well as hydrolysis and biodegradation of organic compounds. Leachate is produced at the time of exceeding the maximum water absorption by the weight of the stored waste. Absorptivity for the compacted mass of waste is estimated to be 30% of volume [2]. Leachate from landfills located in Poland is characterized by high values of COD and BOD5, including high salinity and hardness, high concentration of ammonia nitrogen and relatively low levels of heavy metals. Hardness is related to presence of chlorides and sulfates of calcium, magnesium, potassium and sodium [3]. Wastewater treatment of waste is more complicated than municipal wastewater treatment. This is due to high concentration of pollution found in the leachate. The composition of pollutants varies with age of the landfill. It was necessary to take into account the high concentration of refraction, as well as differences in volume depending on the season. Research carried out for many years, show that it is possible to successfully purify leachate when using one process unit. The methods usually employed for this purpose are hybrid systems that connect biological methods of physico-chemical processes: advanced oxidation processes, membrane methods or adsorption on activated carbon. More and more attention is focused on the use of advanced oxidation processes - AOP. The distinguishing feature of these processes is the presence of an oxidizing agent. Oxidizing agents are very reactive hydroxyl radicals. They have a high oxidation-reduction potential, so that they are able to oxidize organic compounds [4]. AOP processes lead to the mineralization of contaminants. It is based on the oxidation of to carbon dioxide, water, and inorganic compounds [5]. The use of AOP as a pre-treatment, supporting biological treatment, causes that these processes are effective and cost-effective from the point of view of economic development [6]. 1. Landfill leachate - composition and characteristics The volume and composition of leachate is closely associated with many factors, including: • ground conditions, • topography of the environment, • the method of separating substrate of the landfill site and the bed from contact with ground and surface waters, The use of Advanced Oxidation Processes (AOP) for the treatment of landfill leachate • • • • • • 205 the amount of atmospheric precipitation, humidity and the rate of evaporation, the type of collected waste and its degree of fragmentation, biochemical and physical changes that occur in the waste, storage and compression technology of waste, the age of the landfill site, vegetation which grows in the landfill site after its reclamation [7]. Over time, wastes accumulated in the landfill undergo biochemical and physical processes. Characteristic of leachate is closely associated with these processes. There are three main phases of biochemical stabilization of waste: • aerobic, • anaerobic acidic, • methanogenic. Simultaneously with these there a (...truncated)