Chloride
0 5,000 10,000 15,000 20,000 25,000
Jan-93 Jan-94 Jan-95 Jan-96 Jan-97 Jan-98 Jan-99
concentration in mg/l
pH value
6.6 6.8 7.0 7.2 7.4 7.6
Jan-93 Jan-94 Jan-95 Jan-96 Jan-97 Jan-98 Jan-99
pH units
COD
0 200 400 600 800
Jan-93 Jan-94 Jan-95 Jan-96 Jan-97 Jan-98 Jan-99
concentration in mg/l
TKN
0 10 20 30
Jan-93 Jan-94 Jan-95 Jan-96 Jan-97 Jan-98 Jan-99
concentration in mg/l
Zinc
0 1,000 2,000 3,000 4,000
Jan-93 Jan-94 Jan-95 Jan-96 Jan-97 Jan-98 Jan-99
concentration in ug/l
Cadmium
0 500 1,000 1,500 2,000 2,500 3,000
Jan-93 Jan-94 Jan-95 Jan-96 Jan-97 Jan-98 Jan-99
concentration in ug/l
Lead
0 20 40 60 80
Jan-93 Jan-94 Jan-95 Jan-96 Jan-97 Jan-98 Jan-99
concentration in ug/l
EOX
0 1 2 3 4 5
Jan-93 Jan-94 Jan-95 Jan-96 Jan-97 Jan-98 Jan-99
concentration in ug/l
Figure 2.11 Site D leachate quality data from cell containing MSW incinerator fly ash and APC residuesin big bags
2.5.2 Overview of leachates from MSW incinerator fly ash/APC residues
• The leachates contain high concentrations of dissolved mineral salts, with TDS typically in the range 50,000 to 150,000 mg/l.
• Chloride is the dominant anion, with sulphate much lower, often as low as ~1000 mg/l or less.
• The dominance of the cations depends upon the type of scrubbing process: calcium is the major cation in the leachate if the residues are from a dry or semi-dry process. Sodium and potassium greatly exceed calcium in leachates from wet scrubbing process residues.
• Concentrations of major components are often spikey, showing extreme fluctuations from one sample to the next. The reasons for this are not known but may be due to dual phase hydraulic characteristics of the materials (as with MSW incinerator bottom ash) and the management of the leachate collection system (saturated or unsaturated).
• pH values are predominantly near neutral, but occasional spikes up to pH 11 suggest that strongly alkaline conditions may persist within the waste matrix. The erratic values in collected leachates and the predominance of neutral values may be due to a combination of carbonation in the leachate collection system (and perhaps along major flow channels in the wastes) and the two phase flow behaviour noted by Johnson et al. (1998) for bottom ash.
• Leachate may contain COD concentrations of several hundred mg/l. No corroboration by TOC analysis has been found, and interference from the high chloride concentrations cannot be completely ruled out. However, the levels are consistent with those reported by Hjelmar (1996).
• No information (e.g. BOD results) has been obtained on the nature of any organics present. Some trace organic analysis has shown that PAH, BTEX and EOX are all very low, typically sub-µg/l, while oil is <1 mg/l.
• The leachate TKN is typically several mg/l, peaking as high as ~25 mg/l. One operator found that the TKN is mainly ammoniacal nitrogen.
• High concentrations of Pb and Cd are present:
- Pb highly variable, greater in semi-dry process (~1000 mg/l), lower in wet process residues (≤10 mg/l);
- Cd typically several tens of µg/l, up to several hundred µg/l, and occasionally at mg/l levels. The field data from Site E show similar concentrations from semi-dry and wet-scrubbed residues.
This is in contrast to the ranges reported by Hjelmar (1996) for leaching test eluates, which indicated sub-µg/l concentrations from wet-scrub residues. This may be due, in part, to the absence of carbonation, and consequent high pH values, in laboratory leaching tests;
heavy metals in general may be very variable in collected leachates, due to the contrasting redox and pH conditions that may develop in the leachate collection system, flow channels within the waste mass and in interstices between the waste.
• None of the sites visited showed any evidence of gas generation or other evidence of biological activity in cells receiving fly ash and APC residues.
• No field data have been obtained for L/S ratios >0.25.
• No analyses have been found for some parameters of interest to this project, including oxidised nitrogen and trace organics.
2.6 Proposed leachate source term values
The data set from the Phase 2 study (Environment Agency, 2004) is generally consistent with the chemical analyses reported from both UK and European incinerator residues. Kappa values were estimated for raw (untreated) and carbonated bottom ash and APC residues, using the comparison of leaching test results at LS1 (column) and LS10 (batch tests). The kappa value is derived from:
Kappa value = ln [LS1] - ln [LS10]/10 - 1
For some parameters where data are missing, kappa values have been estimated. For the acid neutralised APC residue, no column tests were done so the kappa values for carbonated waste have been used.
The maximum concentration, Co, has been estimated from the kappa values and either CLS10 or CLS1. The spread of the data from the three incinerators provides a high end and low end estimate of the maximum value of Co. The high end estimate is taken as the maximum point of the PDF for each parameter. The low end estimate has been divided by the hydraulic factor of 5, to give the minimum point of the PDF. The most likely value still has to be evaluated, by reference to the shape of the PDF in the existing LandSim model.
For most APC residue parameters, the average Co values were very similar to the maximum values obtained from lumped Site E data (see Section 2.5.1). This provides some justification for the use of the kappa values derived from the experimental results. The proposed source term data for raw bottom ash and APC residues are summarised below and full details are provided in Environment Agency (2004).
2.6.1 Bottom ash (untreated)
The proposed source term data presented in Table 2.6 below is derived from a limited number of laboratory tests using L/S10 batch leach tests and a recirculating upflow column test at L/S1. This information should be used to inform professional judgement when carrying out a groundwater risk assessment for a particular landfill site.