Solid Waste & Recycling

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Leachate Treatment

A system effective enough for onsite effluent disposalThe Clearview Landfill in Stayner, Ontario has efficiently treated landfill leachate onsite since 1996 using the Waterloo Biofilter. The system i...


A system effective enough for onsite effluent disposal

The Clearview Landfill in Stayner, Ontario has efficiently treated landfill leachate onsite since 1996 using the Waterloo Biofilter. The system is effective enough to allow the effluent to be disposed onsite in subsurface trenches, the first site in Ontario to do this. Contaminants are efficiently removed despite fluctuating concentrations, and with no added nutrients.

Laboratory and field research obtained at the Clearview landfill has yielded valuable information with respect to the removal of: biochemical oxygen demand (BOD), chemical oxygen demand (COD), total suspended solids (TSS), iron and other metals and volatile organic compounds (VOCs). Cost recovery for this installation took less than three years.

The technology

The biofilter is a single-pass, absorbent trickle filter developed in 1991 at the University of Waterloo. It uses an absorbent plastic foam medium to optimize retention time and biochemical treatment in just 10 per cent of the area required for conventional sand, soil or peat filters.

Raw leachate is sprayed onto the biofilter in a rougher-polisher sequence and is biochemically treated as it percolates down through the medium. Following an acclimatization-construction period, the treatment results are excellent (see Tables 3 and 4). Treatment insulates leachate manholes and avoids excessive hydraulic spikes. It’s barely affected by natural flow and concentration variations. A robust data collection and remote monitoring system troubleshoots and provides process optimization.

Filter media for sustainable biological degradation should combine high surface area for microbial attachment with large pores so that the microbial slime does not plug the medium and short-circuit the treatment process. High absorption provides microbes adequate contact time to degrade the contaminants. Typically there is a trade-off between the higher retention time of sand with a higher rate of plugging, and the lower retention time of conventional trickle filter media with lower plugging potential. However, the filter medium forms an interconnected, three-dimensional reticulate solid framework with high surface area, high porosity, high absorption and dual pathways for air and wastewater.

Lab and field studies

Following successful column experiments in 1993 on three leachates, a rougher-polisher sequence of filters was tested in 1995. Following the “start-up,” high removal rates of 99 per cent BOD5, 94 per cent TSS, 97 per cent total nitrogen (TKN, as ammonium or organic nitrogen, not including nitrite or nitrate), 93 per cent Fe, and 99 per cent turbidity (NTU) were obtained. Nitrite formed in the first 10 days and thorough nitrification and 40 per cent TN removal followed in 30 days. Typically, pH increases from 7 to 8.9.

The installation in Stayner was constructed to treat 16 m3/day of leachate. The unit consists of three concrete tanks to serve as a roughing filter (36 m2 area total, 2.4 m deep) and two tanks to serve as a polisher to treat the rougher effluent (24 m2 area total, 2.4 m deep).

A dependable data collection and remote monitoring system allows temperature and operational parameters such as pump on-off cycles, cumulative pump times, alarms and flow rates to be reviewed remotely. Data is downloaded on demand from any of the administrative or maintenance computers and the station file is automatically updated.

The treatment plant was started up in mid-1996 and within a month the effluent was light clear yellow. BOD removal was greater than 90 per cent although nitrification had not yet started. Leachate treatment deteriorated in early 1997 due to cold temperatures (below 5C due to exposed manholes and ongoing construction), but by early spring the effluent was clear yellow again. By summer 1997 complete nitrification (>99 per cent) and excellent removal of BOD (98 per cent), TSS (98 per cent), and COD (93 per cent) were accomplished. Rapid pumping of excessive volumes of leachate into the plant from other cells caused short-term treatment lapses, but this was resolved by regular pumping schedules.

Construction and operational difficulties were solved by September 1997, and the first of the four time divisions (September 1996 to August 1997) represents the “start-up” period before the ongoing treatment phase. The second and third divisions represent summer and winter periods, and the fourth a “four-season” period. Removal of BOD, TSS, and COD are fairly consistent through the seasons, but TKN removal noticeably decreases during the winter, due to the sensitive nitrifying bacteria..

Removal of VOCs (>99 per cent) and phenols does not suffer during winter. Toluene is the most common VOC, with some m+p xylene, o-xylene, and little benzene and ethylbenzene. Others are typically non-detectable.

The three roughing filters have iron staining and carbonate precipitation on the upper surfaces as well as a heavy oily odour in them when the access hatches are opened. The distribution nozzles plug up occasionally with bituminous-like residue and are cleaned every two months. The two polishing filters are relatively clean with no oily odour. The water analyses show that hydrocarbons and VOCs are removed in the roughing filter, as are most of the metals and other constituents. Analysis of the calcareous crust forming in the rougher filter indicated that the metals are being removed from the wastewater stream. Iron, manganese and lead are metals of concern which are readily removed in the filter. Other metals of concern (such as cadmium) are typic- ally present in very low amounts (< 5 g/L).

After hydrogeologic studies were conducted on the effects of chloride and nitrogen on the groundwater, environment ministry approval was granted for disposal of the treated effluent in a raised and pressurized leaching field on a permanent basis. Since August 1997, hauling to sewage treatment plants has not been required of the treated cell, and substantial leachate from the adjacent cell has also been treated. The plume from the disposal bed shows elevat- ed nitrate and chloride levels, but no addition of BOD, COD or metals.

The system works well in winter and summer, but performs better when the wastewater flow rates are varied gradually and when the leachate manholes are insulated from the winter cold.

Written by Craig Jowett, Ph.D., P. Eng., president of Waterloo Biofilter Systems in Rockwood, Ontario.

Table 1. Biochemical analyses of laboratory leachate tests (May to July 1995, all mg/L). n = number of samples
Parameter n IN OUT % Parameter n IN OUT %
BOD5 5 201 3 99 TN 5 172 102 40
TSS 6 84 5 94 pH 7 7.04 8.85
COD 6 460 125 73 Fe 5 9.4 0.6 93
TKN 5 171 4 97 NTU 5 108 1 99

Table 2. Metals analyses of laboratory leachate tests (May to July 1995, all g/L).
Metals n IN OUT % Metals n IN OUT %
Zn 5 23 13 43 Mo 5 6 4 38
Cu 5 9 32 ++ V 5 31 15 53
Ni 5 26 29 0 Sr 5 1532 395 74
Pb 5 50 30 40 Be 5 0.5 0.3 42
Cd 5 3 2 41 Ag 5 20 10 50
Mn 5 1273 11 99 Ba 5 387 25 94
Fe 5 14,441 104 99 Ti 5 20 4 82
Al 5 177 34 81 B 5 1828 2186 +
Cr 5 8 7 13 As 5 2 2 11
Co 5 9 5 49 Se 5 2 1 30

Table 3. The summary of field analyses taken from the raw leachate and polisher effluent (1996-1999).
Parameter Sept 96 – Aug 97 Sept 97 – Oct 97 Nov 97 – Apr 98 May 98 – Jan 99
(mg/L) n IN OUT % n IN OUT % n IN OUT % n IN OUT %
BOD5 5 877 187 79 7 1239 14 99 32 287 17 94 8 427 21 95
TSS 6 111 25 77 6 208 3 99 32 111 5 95 8 72 2 97
COD 5 935 450 52 6 1766 88 95 32 559 139 75 7 859 258 70
TKN 2 182 87 52 4 151 5 97 27 72 12 83 8 164 6 96
TN 2 182 170 7 4 151 69 54 30 75 48 36 8 171 54 68
VOCs (g/L) 2 47 0 >99 3 332 0 >99 16 191 0 >99
Phenols (g/L) 2 18 4 78 1 275 1 >99 2 0.2

Table 4. The averages and ranges of influent and effluent during the entire period after “start-up” (September 1997 to January 2000, low flow days omitted).
Parameter (mg/L) n IN IN Range OUT OUT Range % Removal
BOD5 53 421 35-2595 17 4-39 96
TSS 52 110 18-638 4 0-13 96
COD 51 725 92-3770 170 16-741 77
TKN 45 105 33-216 10 1-26 90
TN 48 107 41-218 55 31-173 49
VOCs (g/L) 19 213 29-534 0 0-1 >99

Table 5. Analysis of the calcareous crust forming in the rougher filter indicates that the metals are being removed from the wastewater stream (December 1998).
Metal mg/L Metal mg/L Metal mg/L Metal mg/L
Ca 70.5% Na 0.1% Al 290 Zn 28
Fe 4.25% S 0.1% Ba 253 V, Sn 12
Mg 0.3% Sr 526 P 206 Ni 4
Mn 0.2% K 291 Pb 80 Zr 1
Ag, As, Be, Bi, Cd, Ce, Co, Cr, Cu, Hg, La, Mo, Nb, Sb, Se, Te, Th, Ti, U, W, Y are all <1 mg/L

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