Investigations of the solid waste bio-dried fraction obtained in mechanical - biological treatment plants

The Polish legislation acts issued in the years 2012 and 2013 have introduced a number of important targets in waste management [1], [2], [3] promoting mechanical–biological treatment. There are two types of MBT technologies: the mechanical biological pretreatment (MBP) and the mechanical biological stabilization (MBS) [4]. Both of these are processes of which the main target is to minimize the amount of solid waste to be landfilled. The simplest way to reduce the amount of waste deposited in landfills is the Mechanical - Biological Pretreatment (MBP), the idea of which is presented in figure 1. The Mechanical- biological stabilization (MBS) is also the typical method for the production of refuse-derived fuel (RDF) based on municipal solid waste (MSW) as a source. In the fractions generated in the MBS processes, there are a high caloric fraction existing in the majority and a small amount of inerts. The stabilized 0-20 mm fraction can be used on landfills as an intert fraction if the measured parameters (LOI, TOC, AT4, heat of combustion) correspond to the requirements given in [3] and [5]. In The Decree of the Minister of Environment [3] the limiting amounts of  these parameters are: the oxygen uptake AT4 < 10 mg O2/ g d.m., LOI < 35 % d.m., TOC < 20% d.m. While in the Decree of the Minister of Economy [4] TOC < 5% d.m., LOI < 8% d.m. and the maximum value of the heat of combustion 6 MJ/kg d.m.


Figure 1. A comparison of MBP and MBS technologies, according to [4]

In accordance with EU legislation, it was introduced as a new basic law [1] which indicates the conditions to be fulfilled for the purpose to convert the status of waste tied with refuse - derived fuel  (RDF) to the status of a desirable product. The recovery processes allowing to obtain such a desirable product may be realized in many ways. These are the following methods of waste treatment which aim to achieve a desirable product, like fuel: aerobic, anaerobic, mechanical and biological treatment (MBT). The main concept of MBT is the combination of mechanical treatment technologies (screens, magnets, etc.) with biological treatment (composting, anaerobic digestion, biodrying) [6,7].
Biodrying technology (MBS) focuses on the production of stabilat which is used in the production of solid recovered fuel (SRF). In an increasing number of technologies MBS shows that waste is appreciated as real energy sources. The achievement of a high calorific product can be done by the removal of the maximum amount of water contained in the matter with the minimum loss of the organic carbon. This being the source of the calorific value [4]. The reduction of the volatile substances and inert components determines a decrease in the negative environmental impact. According to the law criteria, the so called “stabilat”, which is the resulting material after the bio-drying processes taken from MBS reactors, should be characterized by the total organic carbon (TOC), loss in the ignition (LOI) and respiration activity (RA-AT4). The reactivity/stability of the MBT products can be determined by aerobic and anaerobic methods [8]. The aerobic indices are based on respiration techniques, while the anaerobic- on biogas production. Respiration techniques are based on the measurement of the loss of O2 in the reactor and are the most accepted because of the precise information about the real biological activity of the samples [8]. The methods based on the O2 uptake rate have been classified into: dynamic (with a continuous supply of air) and static (without a continuous supply during the assay) [9]. For the measurement of the respiration activity the following equipment is used in the static methods: Sapromat, Oxitop, Lutron 5510 and in the dynamic methods: Costech, Micro-Oxymax [9,10].
2. Materials and methods
The materials were taken from an MBS industrial plant. The samples were bio-dried municipal solid waste (MSW), obtained post seven days processing in MBS technology. After the biodrying step, the stabilat went to the screen to separate the 0-20 mm and 20-80 mm fractions. The object of the study was the 0-20 mm fraction, separated from the others. Three kinds of samples were collected: the first was from the front part, the second from the middle part and the third from back part of the reactor. The probes were taken according to the standard procedure [11].
Nine small samples (5 g mass), three samples per each part of the reactor,  were characterized by its humidity [12] and LOI [13]. The respiration activity was measured by a respirometer Oxymax ER-10 (Columbus Instruments) which detects the volumes of gas consumption (O2) and production (CO2) by the probe. Approximately 200 g of each sample were determined in the Oxymax, which is a highly automated system and performs the measurements in a closed gas sensing loop. The respirometer took a series of gas measurements every 5 minutes, recording the net increase or decrease in the concentration of the monitored gases.
3. Results
The results obtained by the determination of the loss on ignition (LOI) and the dry matter and humidity are illustrated in table 1. The mean values of the humidity are presented accordingly: front part (numbers: 1,2,3) - 22.06 %, middle part (numbers: 4,5,6) - 25.20 %, and back part (numbers: 7,8,9) - 32.12%.

Table 1 – The humidity and LOI values in the samples after the MBS processing in an industrial plant


Part of reactor

FRONT PART

MIDDLE PART

BACK PART

No of sample

1

2

3

4

5

6

7

8

9

Humidity

21.3

24.4

20.4

25.0

23.3

27.2

35.8

32.4

28.0

LOI

31.3

31.0

31.0

29.0

28.8

31.6

32.2

29.9

27.0

Table 2 – The total amounts of the gases monitored in the Oxymax for the 0-20 mm fraction


PART OF REACTOR

Total consumption 1) O2[mg/94 h]

Total production CO2[mg/94 h]

CO2/O2
[-]

Consumption O22)                                [mg O2/g d.m.]

Front

1186.33

1596.25

1.35

7,90

Middle

1128.15

1678.33

1.49

7,54

Back

1539.69

1828.81

1.18

11,34

1) Total consumption was measured per overall sample of 200 grams of wet material
2) Consumption was measured per 96 h


Figure 2. The monitored gases for the 0-20 mm material from the back part of the reactor

Figure 2 presents the measured biological activity by the amounts of oxygen consumption and carbon dioxide production during 1152 cycles (each cycle is equal to 5 minutes). The graph shows that the concentrations of the monitored gases have still decreased. The curves tendency shows the overall biological stability. Table 2 presents the total amount of O2 consumption and CO2 production for the determined samples (200 g each). The consumption of O2 (tab. 2) was measured per 4 days using the Oxymax  dynamic equipment where the air flow was stable and equal to aprox. 20,8% concentration. These values can be treated as AT4 parameters without a lag-phase.
4. Conclusions
The humidity of the 0-20 mm fraction increases with the depth of the reactor. The materials taken from the back part of the dryer reactor are the most humid and reactive. The consumption of O2 for the back part of the reactor is higher than the requirements in the Decree of the Minister of Environment [3]. It allows to make the conclusion that the reason is the inadequate air circulation in the dryer reactor. According to the Polish law requirements [3], the investigated 0-20 mm fraction had been properly treated in the MBS as LOI < 35%. However it could not be used as an intert material on landfills where the maximum LOI should be not more than 8 % d.m.[5]. This conclusion is similar to the opinion about the 0-20 mm fraction in the  Ministry of Environment guidelines[14].
References

  1. The Act on Waste [uniform text: Journal of Laws of 2013, No. 0, item 21].
  2. The Decree of the Minister of Environment on the levels of mass reduction of biodegradable municipal waste disposal on landfills and the method of calculating the level of the mass of waste reduction [Journal of Laws of 2012, No. 0, item 676]
  3. The Decree of the Minister of Environment on the municipal solid waste treatment in mechanical and biological plants [Journal of Laws of 2012, No. 0, item 1052].
  4. Rotter S.: Incineration: RDF and SRF – Solid Fuels from Waste. Mechanical biological treatment. “Solid Waste Technology& Management”, Vol.1, Publication A. John Wiley and Sons, United Kingdom 2011.
  5. The Decree of the Minister of Economy on the criteria and procedures allowing to depositing wastes of a particular type on landfills [Journal of Laws of 2013, No. 0, item 38].
  6. Bilitewski B., Oros Ch., Christensen T.H.: Mechanical biological treatment. “Solid Waste Technology& Management”, Vol.2, Publication A. John Wiley and Sons, United Kingdom 2011.
  7. Żygadło M: Gospodarka odpadami komunalnymi, Wydawnictwo Politechniki Świętokrzyskiej, Kielce 2002.
  8. Barrenaa R., d’Imporzanob G., Pons´aa S., Geaa T., Artolaa A., V´azquezc F., S´ancheza A., Adanib F.: In search of a reliable technique for the determination of the biological stability of the organic matter in the mechanical–biological treated waste. ,,Journal of Hazardous Materials’’, no. 162 (2009) p.1065-1072.
  9. Gomez R.B., Lima F.V., Ferrer A.S.: The use of respiration idicates in the composting process: a review. “Waste Management & Research”, no. 37/ 2006, p. 37-47.
  10. Binner E., Böhm K., Lechner P.: Large scale study on measurement of respiration activity (AT4) by Sapromat and OxiTop. “Waste management”, no. 32/2012, p. 1752-1759.
  11. Standards Association of Poland (1987). Polish standard: Collection, storage and transmission of waste samples and initial preparation to investigation.BN-87-9103-03.
  12. Standards Association of Poland (1993). Polish standard: Fuel property testing. Determination of the total moisture. PN – 93/Z15008.
  13. Standards Association of Poland (2007). Polish standard: Characterization of waste – Determination of loss on ignition in waste, sludge and sediments. PN – EN 15169: 2007.
  14. Guidelines of the Minister of Environment on the requirements for composting, fermentation and bio-mechanical waste treatment, Warsaw 2008.

Investigations of the solid waste bio-dried fraction obtained in mechanical - biological treatment plants [Електронний ресурс]  / [Żygadło Maria, Latosińska Jolanta, Dębicka Marlena] // Режим доступу: http://eco.com.ua/content/investigations-solid-waste-bio-dried-fraction-...

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