A case study on anticipated leachate generation from a semi-aerobic sanitary landfill at Aruwakkalu (Puttalam District) and its impacts

Abstract

Leachate generation from sanitary landfills is regularly expected in tropical regions whenever the municipal solid wastes (MSW) attain its field capacity. However, there seems to be no studies documenting the behavior of high moisture laden MSW in sanitary landfills constructed or proposed in arid areas. Therefore, in this study prediction of leachate quantities is conducted using the water balance method with reference to a semi-aerobic sanitary landfill that would be designed to dispose 1,130 tons of pre-compacted MSW per day (in 2035) having a high moisture content of 70 % by w/w from the Metro Colombo Region which has a mean annual rainfall of 2,500 mm (but the landfill to be sited in Aruwakkalu; an arid area having a mean annual rainfall of <1,500 mm). This article also discusses the feasibility of leachate treatment (considering the expected quality) and other issues that would arise due to leachate generation. Leachate production occurs during the peak rainy seasons (October; 2nd inter-monsoonal period and November–December; north-east monsoonal period) only despite the fact that the incoming MSW has a high moisture content. Furthermore, the generated leachate is a methanogenic leachate with a low BOD₅/COD (<0.3). At higher leachate heads, leachate breakthrough time and the time of travel (TOT) for Cl⁻ are lower, but seepage velocities and flow rates are higher for both leachate and Cl⁻. Breakthrough time and hydraulic conductivity show an inverse relationship considering groundwater contamination in Aruwakkalu having a silty-sand soil (33–93 % sand), but no proper relationship between breakthrough time and seepage velocity.

Appendix: brief description of the MCUDP

MSW transferring to the Landfill Site and Unloading Operations at the Landfill Site

It has been proposed to construct a transfer station site at Meethotamulla (located north of the Colombo city center) adjoining the currently operated dum** site simply by demolishing the existing windrow composting facility. After compaction the MSW would be transported by rail (using 2 trains per day) to the landfill site covering a total distance of 170 km.

Without any landfill pre-treatment, it has been planned to install a “Compactor cum Loader” under the dum** pad (which would be designed to have a day’s capacity of MSW or 1,130 tons in 2035 as a maximum) with the inlet hoppers opened to the surface of the dum** pad to facilitate easy loading of MSW. Three mini tractor shovels with pneumatic wheels would be operating to load the compactors by pushing the MSW into the hopper holes. The compactors would press the delivered MSW (increasing the bulk density to 750 kg/m³ from 350 kg/m³ with a 10 % moisture loss) and transfer them to a total of 52 steel containers (each container having a waste carrying capacity of 20 tons with the interior coated with an epoxy coating) per day that will be placed on tractor trailers. Accordingly, three compactors are to load 12 containers per hour, hence approximately 2.5 h is needed to load 26 containers for one train. Tractors would be deployed to tow the loaded containers along a tractor lane to the appropriate loading point under a Transfer Crane, which would lift the container and place it on the rail wagon. Unloading of the empty containers from the rail wagon and placing them on the tractor trailers also to be done by the Transfer Crane (there shall be 2 Transfer Cranes to ensure fast and uninterrupted transfer operations).

At the Aruwakkalu landfill site, there will be an unloading transfer station about 400 m from the landfill edge. A transfer crane shall transfer the MSW loaded containers from the rail wagon to the bed of tip** trailers. Also it shall transfer the empty containers from the tip** trailers back to the rail wagons. About 30 tractors would be deployed to tow the tip** trailers to the sanitary landfill. The back door of the containers would be opened and then the tipper bed would be tilted along with the containers to unload the MSW to the landfill cells. Then the tractors shall take the empty containers to a washing bay and then take the containers back to the transfer station. A total of 15 fulltime workers would be stationed to operate the landfill and the transfer station.

Engineering details of the landfill

The proposed landfill is a semi-aerobic landfill (Fukuoka Method) in which leachate is collected in a leachate collection pond through perforated pipes packed with small crushed stones and partial air gets supplied through the collection pipeline on the slope and gas extraction well (Fig. 3).

Fig. 3

Sanitary landfill method (Source: DOHWA Engineering Co Ltd et al. 2014)

It has been planned to open the landfill in 2015 (when there will be 1,010 tons/day from a population of 1,085,700) and end the operations in 2035 (when there will be 1,130 tons/day from an estimated population of 1,254,900). The landfill will be operated in three phases as follows (as landfilling cells and the cells within a phase are separated by inter-cell bunds) and the total lifespan has been designed for 20 years.

• Phase I: total area around 127,755 m² and a cell height of 60 m with 10 years lifespan (to be commenced in 2016).

• Phase II: total area around 90,063 m² and a cell height of 60 m with 7 years lifespan (to be commenced later).

• Phase III: total area around 44,448 m² and a cell height of 45 m with 2 years lifespan (to be commenced later).

It has been planned to introduce a single composite lining system comprising a soil-bentonite clay layer (having a hydraulic conductivity/permeability coefficient ≤1 × 10⁻⁷ cm/s) and then fortify this liner with a HDPE sheet (Fig. 4). The leachate collection and drainage installation plan has the following criteria (Fig. 4).

Fig. 4

Leachate collection and drainage installation plan (Source: DOHWA Engineering Co Ltd et al. 2014)

Leachate collection/drainage layer

• Thickness: 30 cm minimum

• Hydraulic conductivity/permeability coefficient: 10⁻³ cm/s or more.

• Particle sizes of collection/drainage layer: 10/13, 16/32 mm.

• Bottom ground floor slope: 2–4 %.

Leachate collection/drainage pipes (porosity type)

• Minimum diameter of collection/drainage pipes—15 cm or more per US Environmental Protection Agency (USEPA).

• Pipe hole diameter of collection/drainage pipes: 1 cm or larger and smaller than the minimum diameter of collection/drainage pipes.

• Distance between holes: Collection pipe diameter; 1:1–1.5:1.

• Spacing distance between collection/drainage pipes: 15–40 m (50 m maximum).

Leachate treatment, disposal and the quality of the treated effluent

As per the details given in the Feasibility study conducted by DOHWA Engineering Co Ltd et al. (2014), 198 m³ of leachate per day is expected in the period of 2035 and leachate volume (Q) was calculated using the equation Q = 1/1,000 (C1 × A1 × C2 × A2) × I where I, C and A denote rainfall intensity (mm/day), leachate factor (used as 0.5 in the feasibility) × landfill area (127,755 m²), respectively. Giving an allowance of 10 %, the leachate treatment plant has been designed to 220 m³/day. Additionally, untreated leachate would comprise 2,000, 4,000, 500, 1,200 and 6 mg/L of COD, BOD₅, TSS, NH₄ ⁺–N and TP, respectively. However, detailed engineering details of the collection system and the effluent treatment plant especially the reactor sizes (except for the equalization or regulation tank which has a capacity of 5,400 m³ and a hydraulic retention time of 7 days), types and dosages of chemicals to be used and specifications of the electromechanical units that will be installed, methods of sludge dehydration, etc.) are not furnished in the feasibility report, though it is planned to design, construct and commission possibly a batch type effluent treatment plant that will comprise biological treatment (Fig. 5). Details of the biological treatment system are as follows: Under the anoxic conditions, firstly, the leachate nitrate (NO₃ ⁻) comes into contact actively with the denitrified microorganisms and it reduces nitrate to nitrite (NO₂ ⁻) and nitrite to nitrogen gas (N₂), respectively, and then emit it to the atmosphere, which help to remove nitric oxide (denitrification) from the leachate. Under the aerobic conditions, the leachate organic gets oxidized and ammonia goes through a nitrogen oxidization process changing to nitrite and nitrate in turn. By utilizing the aeration facility, the reactor performs aeration so as to bring the dissolved oxygen (DO) to 1–3 mg/L for the purpose of providing the aerobic conditions.

Fig. 5

Proposed effluent treatment plant at the sanitary landfill facility (Source: DOHWA Engineering Co Ltd et al. 2014)

• Major function: To remove organics and nitrogen.

• Hydraulic retention time (HRT): 220 m³/day, over 5 days.

• Operational method: Anoxic and aerobic conditions

• Mixed liquor suspended solids (MLSS): over 4,000 mg/L.

• Emergency (low loads due to flow fluctuations etc.): To operate a single line only.

Figure 6 shows the location where the treatment plant would be constructed and operated as well as the locations proposed to install groundwater monitoring wells. The dehydrated sludge would be landfilled within the landfill site itself.

Fig. 6

Sanitary landfill layout plan showing the location of the treatment plant, different phases planned and groundwater monitoring wells (Source: DOHWA Engineering Co Ltd et al. 2014)

In addition, as per the feasibility study (DOHWA Engineering Co Ltd et al. 2014), the interior of the 52 containers will be washed using five automatically operated high pressure guns (i.e., at least 1,000 L of water would be required to wash one container) and these washwaters along with the sewage generated by the workforce will be directed to the leachate treatment plant. Furthermore, washwaters from cleaning of the wheels of the tractors towing the tip** trailers would be directed to the treatment plant (i.e., 40 L of water would be required to wash one tractor). The number of vehicles passing the cleaning machine is around 30 taking into consideration of the entry amount of MSW (1,130 tons/day in 2035) and the capacity of a transporting vehicle. However, details pertaining to the quality of the effluent expected from the container and tire washing activities are not furnished in the feasibility report.

As per the feasibility study (DOHWA Engineering Co Ltd et al. 2014), treated effluent would be used to supply make up water (i.e., 10 L × 30 tractors with trailers = 300 L/day) and the rest of the treated effluent would be disposed to the Lunu Oya. The effluent would be treated to conform to the tolerance limits for industrial and domestic wastewaters discharged into marine coastal waters (standards are pH of 5.5–9, BOD₅ < 100 mg/L, COD < 250 mg/L and NH₄ ⁺–N < 50 mg/L) under the National Environmental Act No. 47 of 1980 and its amendments (Extraordinary Gazette No. 1534/18 dated 1st February 2008).

Groundwater monitoring plans

In accordance to the feasibility report, groundwater inspection compatible with configuration of the Sanitary Landfill site and topographical conditions will be put in place for periodic observation of groundwater in order to check whether there is any pollution and also to preclude any environmental and public health problems that could arise. To check for pollution of groundwater by leachate, appropriate spots at upper and lower streams will be identified for installation of wells in order to test groundwater pollution.

Figure 6 shows the locations proposed to install groundwater monitoring wells. Note that two wells will be installed during phase I and one well per phase II and III (exact locations where groundwater wells would be installed in phases II and III are not given).