The effect of humic acids on the reverse osmosis treatment of hazardous landfill leachate

https://doi.org/10.1016/j.jhazmat.2011.08.079Get rights and content

Abstract

This study deals with the treatment of hazardous waste landfill leachate with the help of reverse osmosis. The landfill is located in an abandoned brown coal pit in northern Bohemia. The leachate contained 7.2 g/L of dissolved inorganic salts. Among other contaminants were heavy metals, arsenic, ammonia nitrogen and associated organic pollutants, especially chlorinated compounds. A mobile membrane unit (LAB M30) equipped with a spiral wound element (FILMTEC SW30-4040), with a membrane area equaling 7.4 m2 was used for the pilot plant experiments. All experiments were carried out in batch mode. 94% conversion of the input stream into the permeate was achieved by use of a two-stage arrangement. Removal efficiencies of the monitored contaminants in the feed ranged from 94% for ammonia nitrogen to 99% for the two-valent ions. Removal efficiency for total dissolved solids was 99.3% on average.

Due to varying levels of humic acids in the leachate throughout the year, fouling experiments were performed to investigate the separation process under different conditions than those used in the pilot plant. Leachates containing different concentrations of added humic acids were separated using a thin film composite on a propylene membrane. The added humic acids were obtained from samples of contaminated oxihumolite.

Highlights

► We studied treatment of leachate from landfill placed in former open-cast coal mine. ► The leachate was treated by reverse osmosis with emphasis to humic acids content. ► Removal of total dissolved solids from landfill leachate was 99.3% on average. ► Permeate flux and rejection decreased with increasing concentration of humic acids.

Introduction

Treatments based on membrane technology, like biodegradation and chemical and physical methods (chemical oxidation, adsorption, chemical precipitation, coagulation/flocculation or air stripping), are viable alternatives to conventional methods of landfill leachate treatment. All pressure-driven membrane separation processes (microfiltration, ultrafiltration, nanofiltration and reverse osmosis) are used in the treatment of landfill leachate. Reverse osmosis appears to be the method of choice for the purification of landfill leachate, because it separates on an ionic level. Besides removing organic contaminants, reverse osmosis also removes dissolved inorganic salts. Values of the rejection coefficient higher than 99% were reported [1].

One of the first operational applications of reverse osmosis was installed in the wastewater treatment plant in the town of Wijster in the Netherlands in 1986. The two-stage unit employed tubular membrane modules in the first stage and spiral-wound membrane modules in the second stage with total recovery rate of 54%. The permeate from the second stage meets required limits for discharge into natural water streams [2]. Significant advancement in this field occurred with the development of the desk tubular modules, in which flat membranes are placed between desks in a tube. In the last decade, more than one hundred technological systems using reverse osmosis were installed at landfill sites in Northern Europe, North America and the Far East [3]. An example is one of the largest facilities in Europe, a two stage reverse osmosis installation at a landfill in Ihlenberg, Germany. The DT modules in the first stage have a total area of 1147 m2, the second stage uses spiral wound modules totaling an area of 768 m2. Separation efficiencies of organic and inorganic substances fall within the 98–99% range [4]. Reverse osmosis is also used in combination with other technologies, most often with biological pre-treatment, as is the case for landfills in Mechernich and Kolenfeld, Germany [5]. A two-stage reverse osmosis system with a capacity of 500 m3/day and recovery rate of 80% was put into operation in October 2003 at the Changshengqiao landfill in the city of Chongqing, China. The pH of the leachate was adjusted from 6.0 to 6.5 to prevent premature precipitation. After filtration, the leachate was fed into the first stage of reverse osmosis. The second stage permeate was discharged into the river and the second stage concentrate was recycled into the first stage. The concentrate resulting from that first stage was returned to the landfill body. The separation efficiency for most of the components was 99% and removal of certain ions such as Ca2+, Ba2+, and Mg2+ was accomplished with an even greater efficiency of 99.9% [6]. Recovery rates depend on operating pressure, conditions of separation and the composition of leachate. Recovery rates higher than 95% were reported when high-pressure RO units were used at the Ihlenberg landfill in Germany [7].

In the Czech Republic, membrane separation processes are used in many industrial sectors (food, pharmaceuticals, etc.). However, for the treatment of leachate they have mainly been used in pilot plant tests at limited locations. Among the examples of successful application is the decontamination of a landfill in eastern Bohemia (Nový Rychnov). Groundwater here was contaminated by chlorinated hydrocarbons, organochlorine pesticides, nitrites, chlorides, heavy metals and other substances. Reverse osmosis was used here as a final purification technology after stripping and filtration with activated carbon. Another example includes the pilot plant tests performed at a disposal site for fly-ash and desulfurization products from a power generating plant in Prunéřov. The effectiveness of commercial antiscalant to mittigate scaling of calcium sulfate was tested. A membrane unit with the capacity of 5 m3/day and a recovery rate of 80% was used. The average separation efficiency was 99%.

Fouling of reverse osmosis membranes causes significant losses to productivity and adds to operational costs. Organic matter, such as humic acids, have been identified as one of the major foulants for membranes [8]. Humic acids comprise a group of heterogeneous polymeric organics. Their molecular weights range from a few thousand to a few hundred thousand Daltons, depending on their source [9].

Organic matter contained in municipal landfill leachates includes mainly volatile fatty acids and humic substances. Content of the two organic fractions in leachate varies greatly depending on the landfill age. In young landfills, a majority of organic carbon is present as volatile fatty acids [10]. However, in mature landfills, humic substances (humic acids, fulvic acids, and humins) dominate the organic fraction in methanogenic leachate by as much as 60% [11]. Concentrations of humic acids in the range of units to tens mg/L in landfill leachates were reported in previous studies [12], [13], [14]. The content of humic acids is also greatly dependent on the type of deposited waste and the type of landfill, so concentration in hundreds mg/L are presented in some studies [15], [16].

There are a limited number of systematic studies on the fouling of reverse osmosis membranes by humic acids [17]. However, most previous studies focused on fouling during the separation of model solutions. The objective of this study is to investigate the effects of humic acids on the separation of a complex solution, which is represented by the hazardous landfill leachate.

The investigated landfill is located in northern Bohemia and is used for waste management of both hazardous and non-hazardous wastes. The landfill has been in operation since 1993. It consists of two disposal sites located in an abandoned open-cast brown coal mine. The landfill has a total capacity of 1.7 million m3. According to a hydro-geological survey, the natural circulation of water around the landfill has been significantly disrupted. The leachate flows through a drainage system into a central shaft, then pumped into concrete cofferdams located outside the landfill body, and subsequently fed to the city wastewater treatment plant. The daily production of leachate averages 80 m3. The leachate contains large quantities of total dissolved solids (about 7 g/L). Other contaminants include heavy metals, arsenic and associated organic pollution. The leachate composition varies minimally during the year, primarily depending on rainfall. Upon its closure, the landfill will be insulated with the aid of sealing elements so as to prevent infiltration of rainwater into the landfill body, and the surrounding area will be rehabilitated and reclaimed. Given the high percentages of inorganic salts and biologically recalcitrant organic compounds, the leachate cannot be purified in a municipal wastewater treatment plant. Therefore, the site managers are considering the introduction of reverse osmosis technology that would remove most of the contaminants. The purified stream, depending on its quality, would be either drained into the public sewer system or discharged directly into a water recipient.

The possibility of temporary deposition of approximately 30 000 tons of contaminated oxihumolite (is young weathered lignite with low calorific value and high sorption capacity) on the bioremediation site next to the landfill appeared after the pilot plant experiments. Leachate from this site is drained into the same shaft as the landfill leachate, thus affecting its composition. In the past, 23 000 tons of contaminated oxihumolite from nearby highway construction was stored there. This procedure temporarily increased the content of humic acids in landfill leachate by almost three times (from 14 mg/L to 40 mg/L). The original content of humic acids in the leachate is partly caused by leakage of the landfill body, which is infiltrated by ground water from an abandoned open-cast brown coal mine and it ranging between 11 and 20 mg/L in the last 5 years.

Section snippets

Pilot plant experiments

A series of laboratory tests to investigate the effects of various pretreatment methods on the separation process and the quality of the permeate preceded the reverse osmosis pilot plant tests. Pre-treatment included filtration, pH adjustment to acidic by addition of HCl, alkaline precipitation with NaOH and CaO, as well as filtration with activated carbon.

Based on the laboratory results, a reverse osmosis unit consisting of two consecutive stages, in which the concentrate leaving the first

Pilot plant experiments

The results of the analysis of individual technological streams are summarized in Table 1. It shows the analysis of the filtrated leachate, the first stage permeate and the second stage permeate and concentrate.

The composition of the first stage permeate shows greater than 99% removal of total dissolved solids (TDS), and more than 97% removal of organic substances, expressed as total organic carbon (TOC). The first stage permeate conductivity was 165 μS/cm. The only indicator that exceeded

Conclusions

This study dealt with application of reverse osmosis for hazardous landfill leachate treatment on a pilot scale, followed by membrane fouling experiments studying the influence of humic acids on the permeate flow and total rejection of leachate components.

The main conclusions for pilot plant experiments is that the separation process decreased the concentration of inorganic salts from 7200 mg/L in the leachate to 50 mg/L in the permeate, which corresponds to 99.3% rejection. The concentration of

Acknowledgement

This work was supported by the Ministry of Education, Youth and Sport (Project No. MSM6046137308).

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