3.6.1 Forecast modelling for target variables
The cumulative landfill leachate emission was calculated by treating the leachate quality/quantity as a function of temperature and the annual L/S ratio. When the temperature is constant, the relationship between the target components and the L/S ratio can be abstracted as follows:
Leachate:
LFG:
where
y represents the concentration of the target component, e.g. COD, NH4-N, the methane generation rate and so forth,
l represents the L/S ratio and a, b, c are the coefficients.
a, b and c are determined by using linear regression analysis. According to the residual analysis, the simulated range can be reasonably extended based on this model. To simplify the modelling,
aebl
y
c al
y b
the simulation is started from the peak value during the acidic phase and the short initial aerobic phase is omitted.
The L/S ratio in the equation refers to fresh water addition to the system. Or, alternatively, in the case of nitrogen leaching, it can refer to the addition of leachate with full nitrification in the pretreatment step (discussed in more detail later).
With a constant L/S ratio rate, the relationship between the target variables and temperature is estimated via the temperature coefficient, ©:
where
T represents the target temperature [°C],
y represents the concentration of the target component, e.g. COD, NH4-N, the methane generation rate and so forth, at the target temperature,
t represents the reference temperature [°C] and
yt represents the concentrations of the target component, e.g. COD, NH4-N, the methane generation rate and so forth, at the reference temperature.
3.6.2 Forecast modelling for the leachate emission integrator
Based on the emission limit values, with the forecast model it is possible to evaluate the length of time needed for the leachate to meet the direct discharge requirement. Some variables, for example NH4-N, will take a longer time to descend to the target level, which means that they will have heavier impacts on the length of landfill aftercare. Hence, the weight coefficient of each target variable is assumed accordingly. The leachate emission integrator LE is defined as:
where
yi represents the concentrations of the target components, i.e. COD, NH4-N and Cl- (representatives of organic and inorganic components),
Yi represents the emission limit values for the target components and
) (T t
yt
y
T
¦
ii i
i Y
LE
D
y¢i represents the weighting coefficients for the target components.
When the concentration of a target component meets the emission limit value, the value of
is 1 and the value of is α, which is the minimum level.
3.6.3 Leachate management scenarios
The integrators are applied and compared in six different forecast scenarios (Table 11). These landfill scenarios represent various landfill sizes and annual L/S ratios based on the general condition of existing sanitary landfills. The landfill filling period is a 20-year period; no leachate recirculation system will be used before landfill closure. The conversion between the achieved L/S ratio and real landfill time refers to a typical landfill with 200 mm of annual natural infiltration. The selection of the temperature range depends on the landfill size. The annual L/S ratio rate was selected accordingly. For big landfills, an annual L/S ratio rate of 0.013 corresponds to a one-fold infiltration, whereas 0.038 corresponds to a threefold infiltration (extra water 400 mm /year). An annual L/S ratio of 0.076 includes 700 mm/year of fresh water and 300 mm/year recirculation of treated leachate effluent. For medium-sized landfills, an annual L/S ratio rate of 0.034 corresponds to a one-fold infiltration, whereas other infiltration rates are similar to those of big landfills. The use of fresh water, such as surface water collected in the landfill area, is aimed at chloride washout and the recirculation of treated leachate effluent is related especially to nitrogen.
i i
Y y
i i
i Y
D y
Table 11 The general specifications of the six scenarios
Recirculation Water infiltration, mm/year Annual, L/S (L/kg TS/year)
Temperature, oC Big landfill, average waste height: 25 m (area 50 ha, 10 Million tons, 630 kg TS/m3)
Scenario A No Natural infiltration, 200 0.013 32
Scenario B Yes Natural infiltration, 200 + Fresh water recirculation, 400 = 600
0.038 32
Scenario C Yes Natural infiltration, 200 + Fresh water recirculation, 700 + on-site treatment process effluent 300 = 1200
0.076 32
Medium-sized landfill, average waste height: 10 m (area 25 ha, 1.9 Million tons, 580 kg TS/m3)
Scenario E No Natural infiltration, 200 0.034 20
Scenario F Yes Natural infiltration, 200 + Fresh water recirculation, 400 = 600
0.103 20
Scenario G Yes Natural infiltration, 200 + Fresh water recirculation, 700 + on-site treatment process effluent 300 = 1200
0.207 20
Scenarios B, C, F and G may need some amount of leachate recirculation.
3.6.4 Landfill leachate management strategy model
The above work will produce results that describe the status of the landfill leachate in a comprehensive profile, which is combined with different treatment alternatives for the evaluation of applicable strategies. On-site and off-site treatment alternatives included in this study are shown in Figure 9.
An on-site biological leachate treatment process has been proven to be feasible from both technical and economic points of view in previous studies, and it can be applied before the leachate recirculation or as a pretreatment process before indirect discharge. The biological process (nitrification, partial denitrification) can also be adopted with/without post DN or GAC filtration to meet the direct discharge limit value.
In the unit cost estimation, a 10 – 30 year depreciation time and a 6% interest rate have been used (Wang et al. 2009). The estimated future costs of discharge for a municipal WWTP are based on the current unit costs of leachate components and the flow rate. The water infiltration costs have been estimated at 3900 €/ha/year, including investment and operation (Okereke, 2002). When the six scenarios are compared, the aftercare time period comes to hundreds of years and the total costs are calculated based on zero interest rates.
Figure 9 Leachate treatment options for cost estimation (*advanced treatment needed when effluent COD does not meet the limit; **± 25% unit cost changes used for cost estimation)