Long-term behaviour of MSOR in a flushing bioreactor cell

In 1996 an integrated research programme by VAM began to examine the pre-treatment of MSOR in a flushing bioreactor. The objectives of the research were to investigate whether this would provide an alternative to landfilling of MSOR, which recent changes in Dutch waste policy were likely to prevent after the year 2000.

The concept of “flushing bioreactor landfills”, or “enhanced fermentation cells” has been discussed extensively during the last decade. Although technologies adopted vary, a common feature is the recirculation of leachate, to achieve the enhanced and more complete biodegradation of waste materials, coupled with the flushing of pollutants from the waste mass.

The integrated research programme is summarised by Oonk and Woelders (1999a; 1999b), and has three parts:

1. enhanced treatment of MSOR at laboratory scale in 130 litre columns (Vroon et al., 1999);

2. full-scale demonstration of the bioreactor in a 50,000 tonne test cell (Woelders and Oonk, 1999;

Oonk et al., 2000); and

3. a desk study of the characteristics of the final bioreactor product, where options are evaluated (van der Sloot et al., 1999).

The laboratory-scale research focussed on whether the objectives of the larger-scale demonstration project can be attained (degradation of organic wastes to gas, improvement in leachate quality,

biological stabilisation of MSOR). It also assessed whether additional post-treatment is necessary (e.g.

flushing with clean water, post-treatment of MSOR residues), and predicted the final product of MSOR after treatment in a bioreactor.

Four test columns (diameter 300 mm, height 2.1 m) were filled with MSOR at a density of 1.2 t/m³, and placed in rooms at 38ºC. Three of the columns (1, 2 and 3) were operated in a saturated manner, with upward flow of leachate that discharged at the top. (Initially, the infiltration was started at the top, downwards, but due to low permeability this failed to achieve desired rates, and after 80 days the upward-flow system was adopted). Column 4 remained in a downward flow mode, to function as a reference column.

During the initial methanogenic phase of about one year, leachate was infiltrated at a rate of 3000 mm/yr. After this phase (day 376), column 1 was flushed with clean water at a rate of 7400 mm/yr, to accelerate flushing of contaminants, and improve final leachate quality. Composition of the MSOR used was broadly similar to that shown earlier in Table 3.1, with 40 percent water, 26 percent dry organic matter, and 10 percent glass/stones.

The upwardly leached columns produced leachates containing high levels of volatile fatty acids during the first 40 – 50 days, and these were flushed out of the columns instead of being converted to biogas.

Figure 3.4 below shows the cumulative production of biogas, (typically containing 57-63% methane by volume), for the 4 columns, showing that maximum generation rates for column 4, in downflow mode, were typically less than one eighth of those achieved in the upward flow columns.

Figure 3.4 Cumulative gas production, (Nm³ gas per tonne dry organic material) for the four test columns

Flushing of column 1 after completion of the methanogenic phase (after day 376) significantly reduced the concentrations of all contaminants by factors of from 2 – 30. This process is shown in Figure 3.5 below, for COD, ammoniacal-N, chloride and conductivity, and in Table 3.2 for a wider range of

contaminants during the methanogenic and flushing phases. Results cover a period of about 437 days in total, during which a total of about 218 litres of leachate, plus 87 litres of clean water, had been used to flush a mass of 139 kg of MSOR, with an initial moisture content of about 40 percent. This

represents an overall liquid/solid ratio of about 5.5 bed volumes of flushing.

Figure 3.5 Composition of leachate from column one, during methanogenic phase (to day 376), and during subsequent flushing phase

Table 3.2 Composition of leachates during methanogenic phase, and after post-methanogenic flushing phase (results in mg/l, heavy metals in µg/l)

Determinand Methanogenic

Mean Range

After post-methanogenic flushing

COD 4950 (4780-5080) 780

BOD5 313 (215-490) 40-80

Ammoniacal-N 2000 (1700-2200) 290

Chloride 7470 (5700-8400) 95

Chromium 273 (120-500) 110

Nickel 300 (210-350) 65

Copper 100 16

Zinc 2300 (1000-3200) 110

Cadmium 0.9 (<0.2-<2) <0.2

Lead 45 (<40-50) <10

Arsenic 14 (<1-34) 3

Mercury 1.0 (<0.1-2.7) <0.1

It is clear that a much greater degree of flushing would be necessary, in order to enable concentrations of organic materials, and of ammoniacal-N, to be acceptable for discharge into surface watercourses.

In addition, it must be expected that the wastes in the bioreactor will retain a continuing potential to generate and release these contaminants, albeit at a reducing rate, as processes of degradation continue.

Vroon et al. (1999) concluded from the lab-scale trials that MSOR contains readily-degradable organic matter with a high gas formation potential, which was higher than initially expected (at 570-610 Nm³ per tonne dry organic matter). During MSOR treatment in the lab-scale flushing bioreactor, it was concluded that anaerobic biodegradation and stabilisation can be enhanced significantly, and leachate quality can be improved in respect of salts and heavy metals, by flushing the stabilised material with clean water at higher rates. However, for “biological” contaminants (such as BOD and nitrogen), the concentrations cannot be lowered adequately, since these compounds are generated subsequently by degradation processes that will continue over periods of years or decades.

The work, nevertheless, was progressed to a full-scale flushing bioreactor cell, in order to provide much larger scale data over an extended period.

The large-scale research cell to investigate treatment of MSOR wastes has been operated by VAM.

Work has been described in detail elsewhere (Oonk et al., 2000; Woelders and Oonk, 1999; Oonk and Woelders, 1999a), and is summarised below.

During June to November 1997 a 49,000 tonne demonstration cell was constructed, filled with MSOR and instrumented, and completely sealed at base and surface using BES and VLDPE liners and cap.

Cell area was 70 x 100 m, with a maximum depth of wastes of 8m. During February 1998, leachate started to be fed into the MSOR via a surface infiltration system, beneath the cap, but the design rate of 30 mm per week could not be achieved. This was partly due to engineering problems from irregular settlement, but primarily resulted from the low permeability of the MSOR, that has been emplaced at a density of 1.3 t/m³. To overcome these problems, water was infiltrated under pressure, through pipes placed lower in the wastes, and rates of up to 15 mm/week could be maintained during the period to July 1999. Over a 2 year period, 7730 m³ of water was infiltrated, representing 1100 mm (about 11 mm/week), or about one third of a bed volume in total. During this 2 year period, about 2.8 Mm³ of gas was produced, about half of the estimated gas potential of 5.7 Mm³, and there were no difficulties experienced in extraction of leachate, although some scaling of recirculation pipework was observed during the first 6 months. At the end of the trial it was clear that moisture distribution remained very inhomogeneous.

Table 3.3 presents detailed composition data for leachate from the VAM test cell, which is complemented by time-series data for a range of contaminants in Figure 3.6.

Table 3.3 Composition of leachate from the VAM test cell containing MSOR, formed during construction and operation of the bioreactor (after Woelders and Oonk, 1999)

Determinand Construction 6 months 15 months

COD 60,700 39,200 19,400

BOD5 42,000 26,000 9,400

Kjeldahl-N 4,700 5,400 4,200

Chloride 4,700 5,700 6,500

PH-value 7.1 7.8 8.2

Chromium 450 670 1,300

Nickel 770 350 450

Copper 64 18 330

Zinc 2,500 180 560

Cadmium <0.5 <0.5 <0.5

Lead 180 28 56

Arsenic <50 210 190

Mercury 0.63 0.35 0.58

(results in mg/l, heavy metals in µg/l)

Leachate quality is in the same order as that reported earlier from the laboratory-scale trials. However, the greater depth of wastes is reflected to some extent in higher COD and BOD values (COD remained at 10,000 mg/l after 2 years), and in very high values of Kjeldahl-N (to above 5000 mg/l). Ammoniacal- N remained above 3000 mg/l at the end of the trial.

Unlike in the laboratory-scale studies, no attempt was made to complete the test cell study by accelerated flushing with clean water, and it seems likely that this would have been extremely problematical. The study has now ended, and the test cell is no longer accessible (having now been buried under several metres depth of wastes).

0 1000 2000 3000 4000 5000 6000 7000

45/1997 06/1998 10/1998 18/1998 22/1998 26/1998 31/1998 12/1999 44/1999

concentration in mg/l

Chloride TKN NH3-N

0 10,000 20,000 30,000 40,000 50,000 60,000 70,000

45/1997 06/1998 10/1998 18/1998 22/1998 26/1998 31/1998 12/1999 44/1999

concentration in mg/l

COD BOD

0 10,000 20,000 30,000 40,000 50,000

45/1997 06/1998 10/1998 18/1998 22/1998 26/1998 31/1998 12/1999 44/1999

value in mg/l or uS/cm

Bicarbonate Conductivity

0 200 400 600 800 1000

45/1997 06/1998 10/1998 18/1998 22/1998 26/1998 31/1998 12/1999 44/1999

concentration in mg/l

Sulphate Calcium

0 400 800 1200 1600 2000 2400

45/1997 06/1998 10/1998 18/1998 22/1998 26/1998 31/1998 12/1999 44/1999

concentration in ug/l

Zinc Nickel

0 400 800 1200 1600

45/1997 06/1998 10/1998 18/1998 22/1998 26/1998 31/1998 12/1999 44/1999

concentration in ug/l

Chromium Arsenic

0 100 200 300 400

45/1997 06/1998 10/1998 18/1998 22/1998 26/1998 31/1998 12/1999 44/1999

concentration in ug/l

Lead Copper

0 10 20 30 40

45/1997 05/1998 08/1998 11/1998 18/1998 21/1998 24/1998 28/1998 31/1998 34/1998 20/1999

concentration in mg/l

Phosphate as P

Figure 3.6 Composition of leachate from the VAM test cell containing MSOR, from November 1997 to July 1999