MBP research at the University of Hamburg-Harburg

Figure 4.19 Concentrations of COD and inorganic-N in leachate from full-scale landfilling of MBP at Lüneburg (Von Felde and Doedens, 1999a)

Figure 4.20 Percentage of COD load remaining in MSW after up to 100 days of anaerobic fermentation, as a function of period of pre-aeration/composting (Control without pre-aeration = 100%)

(after Spendlin, 1991)

Kettern (1993) confirmed this to some extent, with pilot-scale anaerobic lysimeters in a large-scale laboratory experiment. From day 70 to day 330 of the trials, the COD of leachate from the MBP wastes decreased from 15,000 to about 1,500 mg/l – similar to methanogenic leachates from untreated MSW landfills (e.g. Robinson, 1996).

Other relatively early work looked at the benefits of in-situ pre-composting of the bottom layers of waste within a landfill, in order to prevent a too-vigorous acid phase of degradation, even when untreated MSW is subsequently emplaced above this. Figure 4.21 shows results from full-scale landfills, where such techniques were tested.

Figure 4.21 COD and BOD values in leachate from full-scale landfills employing compaction of the waste (a) and recirculation (b) and as alternatives pre-composting (c) and pre-composting combined with leachate recirculation (d) (after Stegmann and Spendlin, 1986; 1989)

More recent work in which Stegmann has been involved has provided a firmer basis of understanding of the effects of MBP on residual waste characteristics. Recent work by Scheelhaase (Scheelhaase, 2000; Scheelhaase and Bidlingmaier, 2000) included detailed study of the effects of period of composting on TOC and BOD5 values in eluates from the DEV-54 (DIN) test. Figures 4.22 and 4.23 below present these results.

Figure 4.22 Effect of period of composting pre-treatment of residual wastes (wastes) on TOC in eluate from the DEV-54 (DIN) test (Scheelhaase, 2000)

5

Figure 4.23: Effect of period of composting pre-treatment of residual wastes (wastes) on BOD5

in eluate from the DEV-54 (DIN) test (Scheelhaase, 2000)

German legislation demands that TOC values in eluate from MBP wastes in the DEV-54 (DIN) test should not be greater than 150 mg/l, if these materials are to be disposed of into a Class II Landfill.

Extensive treatment is clearly necessary to meet such standards, and doubt remains about how appropriate such 24-hour leaching tests are to check compliance.

It is exceptionally difficult to obtain high rates of flushing at any MBP waste landfill. Experience at Hamburg-Harburg is that emplaced density of composted wastes can achieve about 1.5 tonne per cubic metre, resulting in permeabilities, in the order of 10^-7 to 10^-9 m/sec. Although the high COD and BOD values of early acetogenic phases of decomposition can be avoided by MBP, the long-term COD “tail” in leachates is little reduced compared to that of residual wastes not pretreated biologically. This

demonstrates that although MBP processes can degrade readily-degradable organic waste components, many of these materials which only degrade over longer periods remain essentially unaffected.

A “Sapromat” test can be used to determine the reduction of respiration rate of residual wastes during the composting process. Work has demonstrated that there is no significant difference between

composting in windrows or in closed containers (Leikam and Stegmann, 1999; 1997; 1996; Fricke et al., 1995) (see Figure 4.24).

Figure 4.24 Reduction in waste respiration rates during (a) windrow composting, and (b) container composting, of residual wastes in full-scale MBP plant (Leikam and Stegmann, 1996)

In both cases the respiration rate at the end of composting amounted to 5 mg O2 per gramme of dry matter over 96 hours, which was recommended by the authors to define a biological stable material.

The behaviour of these composted residual wastes when landfilled was modelled in landfill simulation tests, where results could be compared with behaviour of residual wastes not subjected to such pre- treatment (see Figure 4.25 below).

Figure 4.25 COD, BOD5 and total-N in leachates from untreated and treated residual wastes during landfill simulation tests (Leikam and Stegmann, 1999) (NB: liquid:solid ratio at 400 days = 1.6:1, at 700 days = 2.6:1)

For pretreated residual wastes, the acetogenic phase is absent, and after about 250 test days the COD of the leachate is below 1000 mg/l (BOD5 <20 mg/l). A trial period of 250 days corresponds to a flushing period of about 50 years for a 20 m deep landfill, with an annual infiltration rate of 250 mm, or 250 years for a similar site where capping reduces annual infiltration to only 50mm.

A much more significant benefit of pre-treatment becomes apparent when concentrations of total-N (primarily ammoniacal-N) are considered. Whereas the total-N content in leachate from untreated residual waste stabilises at about 1000 mg/l, this value is below 200 mg/l for pretreated wastes. This will result in substantially reduced aftercare costs in terms of leachate treatment, and possibly also in terms of reduced periods of aftercare, although no data have been obtained that can confirm this.

Further studies compared the effects of MBP in terms of total load of contaminants emitted by treated and untreated wastes, and results are presented for COD and total nitrogen blow, in Figure 4.26.

Figure 4.26 Total COD and ammoniacal-N load for untreated, and biologically pretreated (4 months) residual wastes in landfill simulation test. (Leikam and Stegmann, 1999;

Stegmann et al., 1996)

For a liquid:solid ratio of 1:1 in the simulation trials (representing periods of typically from 50 – 250 years at a full scale landfill), the mass transfer of contaminants was about 90 percent less for biologically pretreated wastes than for untreated residual wastes. This is similar to the reduction in gas production noted in similar tests, and corresponds well with the reduction in waste respiration rates noted earlier (Figure 4.24).

Danhamer and Jager (1999) undertook similar landfill lysimeter studies using a range of different MBP wastes from full-scale plants, and compared these with results from an equivalent lysimeter using untreated residual wastes. They provided much more detailed composition data for leachates in the methanogenic phase, which compare well with the results above. Again, the reduction in nitrogen is most pronounced and important (see Table 4.7).

Table 4.7 Data for detailed leachate composition from landfill lysimeters operated by

Danhamer and Jager (1999), containing untreated residual waste, and MBP wastes from a range of German full-scale plants

Location ⁽¹⁾ DA0 DL1 DL2 DL3 QB1 WS1

No. of Samples 10 3 5 7 13 6

Volatile solids (%) 45.9 30.6 31.8 4.2 37.7 21.2

Determinand:

COD 172,000 2,780 1,170 540 4,000 1,900

BOD5 123,000 52 9 158 111 14

TOC 57,000 1,260 450 250 2,100 750

pH-value 6.1 7.5 7.7 9.5 7.5 7.7

EC (µS/cm) 41,000 23,000 21,000 12,000 20,000 16,000

Ammoniacal-N 3,965 197 11 56 292 340

Nitrate-N <0.5 <0.5 <0.5 <0.5 <0.5 <1.0

Nitrite-N <0.5 <0.5 <0.5 <0.5 <0.5 <0.5

Sulphate 4,100 9,500 4,200 2,300 700 5,300

Fluoride <0.5 <0.5 <0.5 0.8 <0.5 <0.5

Chloride 9,100 11,300 6,900 5,700 6,200 4,100

Cyanide (free) <0.01 <0.01 <0.01 <0.01 <0.01 <0.01

Sodium 6,100 4,300 4,500 1,900 3,900 1,600

Magnesium 1,164 268 96 4 226 412

Potassium 3,400 1,500 1,400 900 2,100 1,600

Calcium 6,500 910 320 100 390 380

Chromium 0.41 0.14 0.04 0.03 0.21 0.09

Iron 752 14 4 5 17 16

Nickel 2.10 0.23 0.71 0.16 0.40 0.09

Copper 1.41 0.71 0.80 0.28 0.52 0.18

Zinc 102 3.4 1.0 0.22 1.6 1.2

Cadmium 1.860 0.013 0.016 0.010 0.130 0.022

Lead 0.63 0.31 0.17 0.13 0.46 0.11

Arsenic 0.035 0.013 0.008 0.008 0.010 0.005

Mercury 0.005 0.003 0.003 nd 0.013 0.001

AOX 11.1 1.33 0.73 0.18 1.28 0.97

Weeks composting:

• Intensive nil 4 4 2 16 3

• Secondary nil 9 43 1 8 19

Notes: ⁽¹⁾ • DAO = Darmstadt (untreated residual waste)

• DL1, DL2 and DL3 = Darmstadt and Lohfelden

• QB1 = Quarzbichl

• WS1 = Wittstock

(2) All results in mg/l except pH-value and EC (µS/cm)

Kabbe (2000) published a detailed thesis (in German) at the University of Aachen, that contains much complementary data from pilot-scale outdoor lysimeters containing 1.2 m³ of untreated MSW, untreated residual wastes, MBP residual wastes, and of MSW incinerator bottom ash.

The lysimeters were operated (without temperature control – typically 5 to 15ºC) for a period of nearly 4 years, and produced results similar to those reported above. Table 4.8 contains summary data for a range of heavy metals in the respective leachates.

Table 4.8 Concentration of metals in leachates from test lysimeters containing various untreated and treated wastes (Kabbe, 2000)

Untreated MSW Untreated

MSOR MBP wastes Incinerator bottom ash

Metal units min. max. min. max. min. max. min. max.

Chromium mg/l 0.014 4.18 0.042 0.7 0.008 0.06 <0.0001 0.014

Manganese mg/l <0.01 0.83 <0.01 0.82 0.01 0.69 <0.001 0.09

Iron mg/l 27 513 18 294 31 58 <0.005 8.6

Nickel µg/l 50 1200 60 1200 <20 420 <20 440

Copper µg/l 2 120 <1 66 <1 170 24 220

Zinc µg/l 30 1300 20 4400 10 360 <10 260

Cadmium µg/l <2 358 <1 203 <2 11 <2 70

Lead µg/l <5 360 5 110 <5 600 28 310

Arsenic µg/l 3 42 4 39 <1 34 <2 15

Mercury µg/l <0.5 14 <0.5 71 <0.5 <4 <0.5 4.3

Results from all three lysimeters which contained non-incinerated MSW are similar in terms of heavy metal content. Concentrations of ammoniacal-N (Figure 4.27) are similar for MSW and MSOR, stabilising at about 500 mg/l, but lower for the MBP wastes – at about 150 mg/l, similar to results reported earlier in Figure 4.25.

Figure 4.27 Concentrations of ammoniacal-N in leachates from landfill lysimeters containing