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Chemical Composition and Treatment of Landfill Leachate

Overview

Leachate is liquid that has percolated, due to precipitation or to surface water or groundwater flow, through waste and has :extracted dissolved or suspended material (1). The constituents that make up leachate are widely varied and depend on the landfill :site. Leachate must be treated as either hazardous or non-hazardous wastewater before being released into the environment (2). Many :of the contaminants addressed by the Environmental Protection Agency (EPA) and the Resource Conservation and Recovery Act (RCRA) can :be found in either hazardous or non-hazardous waste landfills.

History of Regulations

Regulations on the maximum contamination levels of leachate constituents are based on the Resource Conservation and Recovery Act of :1976. This act provided a way for the government to monitor and control contamination levels of hazardous waste. RCRA has been :amended since its inception, which have included monitoring and controlling non-hazardous waste as well as minimizing and eliminating :land disposal of hazardous waste (3).

The EPA does not contain any regulations directly addressing the remediation and processing of landfill leachate due to the :extensive covering of the topic in the RCRA Corrective Actions program (3). This program currently has a goal of 95% remediation at :over 3,000 facilities identified through RCRA (6). Remediation and control programs are required during the active life of the :facility and post-closure.

National Standards

The chemical constituents found in leachate can be classified into hazardous and non-hazardous waste (4). The distinction stems from both the level of contamination and the number of contaminants found within the waste. These subcategories only apply to landfills that discharge wastewater from the site.

Hazardous

Hazardous wastewater is defined in Subpart A of the 40 CFR Part 445, “RCRA Subtitle C Hazardous Waste Landfill Subcategory”. :Subtitle C includes extensive classification criteria to identify hazardous wastes, some of which can be found in Figure 1. The :contaminants identified are considered to be dangerous to human, animals or the environment. The levels are given in a one day :maximum as well as a monthly maximum. If a facility violates these levels, they are subject to fines as well as required to take :corrective actions.

Contaminant	Unit	1 Day Max	Monthly Max
BOD5	mg/L	220	56
TSS	mg/L	88	27
Ammonia	mg/L	10	4.9
Arsenic	mg/L	1.1	0.54
Chromium	mg/L	1.1	0.46
Zinc	mg/L	0.535	0.296
Alpha Terpineol	mg/L	0.042	0.019
Aniline	mg/L	0.024	0.015
Benzoic Acid	mg/L	0.119	0.073
Naphthalene	mg/L	0.059	0.022
P-Cresol	mg/L	0.024	0.015
Phenol	mg/L	0.048	0.029
Pyridine	mg/L	0.072	0.025
pH		6.0-9.0	6.0-9.0

Non-Hazardous

Non-Hazardous wastewater is defined in Subpart B of 40 CFR Part 445, “RCRA Subtitle D Non-Hazardous Waste Landfill Subcategory”. :The criteria for this subcategory can be found in parts 257 and 258 of the 40 CFR. These contaminants do not pose as large of a :threat to the environment or health, and occur in smaller concentrations that those found in Subtitle C. Some of the contaminants :can be found in Table 2.

Contaminant	Unit	1 Day Max	Monthly Max
BOD5	mg/L	140	37
TSS	mg/L	88	27
Ammonia	mg/L	10	4.9
Zinc	mg/L	0.033	0.016
Alpha Terpineol	mg/L	0.12	0.071
Benzoic Acid	mg/L	0.025	0.014
P-Cresol	mg/L	0.026	0.015
Phenol	mg/L	0.2	0.11
pH		6.0-9.0	6.0-9.0

Discharge Rates

The RCRA Corrective Action program is designed to remediate and control the effluent wastewater contamination levels. However, not :all of the standards are currently being met. Table 3 contains the national averages for contamination levels at both the hazardous :and non-hazardous waste sites, as well as the estimated discharge rates for direct discharge facilities (5).

Contaminant	Hazardous Median Concentration (mg/L)	Non-Hazardous Median Concentration (mg/L)	Current Discharge Loads (kg/yr)
Ammonia	268	75-82	79,098
BOD	621	67-240	216,819
COD	1,309	994-1,100	1,072,305
TSS	151	21-137	145,038
Alpha Terpineol	0.096	0.123	112
Benzoic Acid	2.482	0.1	159
Phenol	4.4	0.102	135
P-Cresol	0.144	0.075	68
Chromium	0.036	0.028	50
Zinc	0.1	0.1	389

Because the constituents of leachate vary from site to site, individual actions are taken to lower the national averages. :According to recent RCRA reports, approximately 95% of all sites requiring action have been remediated.

Remediation Techniques

Landfill leachate contains many different chemicals, metals, suspended solids and organic compounds and must be treated in :different ways to ensure a safe discharge. The methods described in Section 8 of the landfill guidelines put forth by the USEPA :describe how to address each of these contaminants.

Best Management Practices

In order to reduce the amount and level of contamination of the leachate produced by a landfill, the EPA has described a set of :“Best Management Practices”(6). These practices include capping closed landfills to reduce precipitation infiltration, reducing the :open face area of active landfills, maintaining a rainwater collection system to separate rainwater from leachate, and prohibiting :certain toxic materials from being disposed of in some landfills (7).

Physical/Chemical Treatment

Similar to the process used in municipal wastewater treatment plants, a series of physical and chemical filtration methods are :used to remove many of the contaminants found in leachate. The first step in the process is equalization, which consists of :maintaining a consistent concentration and flow of leachate from the plant so that previously tested methods can be effective with :a known level of certainty. This is done through the use of large basins in which the leachate is discharged into so that it can :remain well mixed. On average, the detention time for the leachate is about two days. The next step in the process is :neutralization, or the leveling of the pH of the leachate. Different acids and bases are added to the leachate to maintain a level :pH between 6.0 and 9.0. Following neutralization is flocculation, the adding of certain chemicals and polymers to bind the :suspended solids into large enough flocs to settle out of the leachate. After the flocs have formed, the leachate moves to a :clarifier in which the suspended flocs settle to the bottom while the now clearer wastewater flows over a weir. (8)

The wastewater now begins the chemical treatment, starting with chemical precipitation. Different chemicals are added to the :wastewater to precipitate dissolved metal compounds. The most common chemicals used in this process are lime, sodium hydroxide, :soda ash, sodium sulfide, and alum. After precipitation, chlorine and ozone are used oxidize the remaining biological and organic :material. The chlorine is also added to remove the ammonia found naturally in the leachate. The leachate is then run through a :filtration media, which includes sand, diatomaceous earth, granular media, membrane and fabric. (8)

Biological Treatment

The method for treating organic compounds in wastewater or leachate involves the use of microbes, or “bugs”. These bugs are :injected into the wastewater and allowed to digest the organic material through either aerobic (oxygen-based) digestion or :anaerobic (oxygen-free) digestion. The bugs digest the organic material into simple organic acids and then into carbon dioxide and :methane. A common method for landfill leachate digestion is the use of a lagoon. A lagoon is a body of water built into the earth, :and serves as a location for aerobic or anaerobic digestion, sedimentation and clarification. Another method for biological :treatment is the attached-growth biological system. In this system, a microbe substrate attached to a rigid media is allowed to :digest organic material as it passes over it. Once the bugs have digested a certain amount, they break off of the rigid media and a :new substrate re-forms over it. The detached bugs settle out to the bottom of a tank where they are mechanically scraped off. The :last main method in use involves activated sludge, a suspended aerobic microbe that is continuously stirred to maintain suspension. :After a specified amount of time, the liquid is sent to a settling tank where the solids settle out and the clear water can be :removed (9). In all of these cases, a sludge is produced that must be treated as well. The sludge is dewatered using gravity :thickening, pressure filtration or sludge drying beds. Gravity thickening allows the sludge to settle out, leaving a less polluted :wastewater on the top which can be reprocessed. Pressure filtration uses a plate and frame press where the water is physically :driven from the sludge. Sludge drying beds are placed onto drying beds, made of either sand or gravel, and allowed to dry (10). :This is the most economical method when land is available. From here the sludge can either be taken to a landfill or processed :further into fertilizer (8).

References

1) Sharma and Reddy. (2004). Geoenvironmental Engineering, John Wiley and Sons, New York, 968 pp.

2) Canter and Knox. ¬(1990). Groundwater Pollution Control. Lewis Publishers, Inc, Chelsea, Michigan

3) “Resource Conservation and Recovery Act Summary” <http://www.epa.gov/regulations/laws/rcra.html>

4) Christensen, Cossu, Stegmann. (1992). Landfilling of Waste: Leachate. E & FN Spon, London, UK.

5) “Wastewater Characteristics” < http://www.epa.gov/guide/landfills/final/technical_e.pdf >

6) “Corrective Action Overview” <http://www.epa.gov/waste/hazard/correctiveaction/index.htm >

7) “Corrective Action Methodologies” http://md1.csa.com/partners/viewrecord.php?requester=gs&collection=ENV&recid=9004247&q=rcra+regulations&uid=1320104&setcookie=yes

8) McArdle, Arozarena, Gallagher. “Pollution Technology Review: Issue 151” William Andrew, 1988

9) Eckenfolder, Musterman. (1995). Activated Sludge Treatment of Industrial Wastewater. Technomic, Lancaster, Pennsylvania.

10) “Wastewater Treatment Technology Description” <http://www.epa.gov/guide/landfills/final/technical_c.pdf >