Further steps in the standardization of BOD₅/COD ratio as a biological stability index for MSW

Highlights

• BOD5/COD ratio is a reliable parameter for measuring the biological activity of waste.

• BOD5/COD ratio values do not seem to be influenced by the duration of the leaching test.

• BOD5/COD ratio values do not seem to be influenced by the liquid /solid ratio (L/S).

• Filtration of the eluate prior to analysis is not compulsory and may be substituted by centrifugation.

Abstract

In recent decades the definition of standard test methodologies suitable for use in assessing the biological stability of solid waste has become increasingly imperative. To meet this requirement, the BOD₅/COD ratio, measured on waste eluate, has been proposed by Cossu et al. (2012) as a more appropriate parameter than the traditional respirometric indices and biogas production measured directly on solid samples. However, to ensure reproducibility, the parameter should undergo standardization of operational conditions.

Previous studies have demonstrated that: the testing mode (static or dynamic) does not influence test representativeness; the long testing time (>6 h) does not influence BOD₅/COD ratio; COD measured on unfiltered or filtered samples is consistent and significant in both cases.

The main aim of this study was to further promote the standardization of this parameter. A series of static leaching tests on representative samples of five types of waste was carried out under different operative conditions: contact time, liquid to solid ratio and pretreatment.

The results obtained demonstrate: the apparent adequacy of a short contact time (2 h), which is highly preferable and would speed up the procedure; a low liquid to solid ratio (5 l/kgTS) which is recommended as a water saving strategy; the applicability of centrifugation of the eluate prior to analysis which is faster that filtration method.

Introduction

In the field of solid waste management, the control of biodegradable organic fractions is a key factor to be taken into account in any modern strategy.

Particularly with landfilling the presence of putrescible waste is the main cause of adverse environmental impacts (groundwater pollution, global climate changes, odour, fires, mechanical instability explorations).

Strategies aimed at limiting these negative effects include avoidance, separate segregation of putrescible wastes (composting, energy production, biofuels, bioplastics, etc.), and accelerated stabilization of waste either before (MBP) or after landfilling (semi-aerobic landfills, in situ aeration).

However, there are a number of reasons advocating control of the biological stability of organic waste:

• The higher the biological activity the higher the amount of expected emissions from landfilling and related environmental impacts (high organic load in leachates, greenhouse gasses in landfill gas, etc.).

• The biological activity of outputs demonstrates the efficiency of Biological Waste Processing (composting, anaerobic digestion, etc.).

• Odour emissions are generally directly related to the biological activity of waste.

• Measurement of biological activity may be adopted as the main parameter indicating the acceptability of waste for landfilling.

The European Landfill Directive EC/99/31 has set targets aimed at avoiding, or reducing, landfilling of non-stable organic materials; however, no official parameters have been provided or limit values set with a view to measuring biological activity in the residual waste (Cossu and Raga, 2008). Indeed, each individual country has adopted independent biological activity parameters and related exit values, in the absence of any form of standardization.

In recent decades numerous authors have taken into consideration a large number of potential solutions for use in evaluating biological stability of waste. Currently, respiration indices and anaerobic tests based on biogas production are the most widely applied. The former can be carried out under static or dynamic conditions on many kinds of organic waste (Adani et al., 2004, Adani et al., 2006, Barrena et al., 2009), whilst the second group comprises biomethane potential production (GB₂₁) measured over a period of 21 days (Heerenklage and Stegmann, 2005). These methodologies are largely applied today, and several studies have demonstrated a good correlation between the different techniques (Decottignies et al., 2005, Cossu et al., 2005, Ponsá et al., 2008, Wagland et al., 2009). However, these indices feature a series of limitations:

• High cost of respirometers.

• Lengthy duration of anaerobic tests.

• Low representativeness due to the presence of inhibiting or toxic substances capable of modifying oxygen consumption.

• Dilution effect due to the presence of biologically inert organic substances that decrease respiration indices of the waste.

To overcome the above shortcomings, measurement of BOD₅/COD ratio on the eluate obtained from a waste leaching test has been proposed (Cossu et al., 2001, Cossu and Raga, 2008). This method would afford the following advantages (Cossu et al., 2012):

• Use of standard equipment routinely present in any laboratory.

• A simple and cheap procedure.

• Representative of the presence of toxic or inhibiting substances.

• Not influenced by dilution effects due to the presence of impurities in the sample.

• Suitable for use with both coarse and finely shredded materials.

• Short testing time.

A preliminary trial conducted with the aim of standardizing the test was described in a study by Cossu et al. (2012); the conclusion reached was that BOD₅/COD ratio (measured on eluate obtained from a waste leaching test) was not affected by use of either a static or dynamic test condition over an identical contact time. Similarly, test duration did not significantly influence values of BOD₅/COD ratio: a contact time of 6 h is recommended to avoid onset of the hydrolysis and oxidation processes.

Furthermore, sample size does not affect BOD₅/COD ratio. The shredding procedure is lengthy and requires use of appropriate equipment; conversely, in the absence of shredding, preparation of the leaching test is simplified and more rapid. The possibility of working directly with untreated waste makes the BOD₅/COD ratio more competitive than other stability parameters, such as dynamic respiration index.

The BOD₅/COD ratio and BOD₅/COD_soluble ratio were both significantly consistent; moreover, the latter could provide additional information on the biodegradability of the mobile fraction of organic matter present. Therefore, an investigation using only the BOD₅/COD_soluble ratio derived from different equivalent pre-treatments is sufficient.

A leaching test involves a series of factors, including contact time between eluent and waste, temperature, liquid to solid ratio (L/S), static or dynamic conditions, type of liquid, kind of waste, etc. In general, leachability also depends on other physical parameters (homogeneity, particle size, porosity, permeability of the solid phase), as well as on pH and redox conditions (Parodi et al., 2011).

Therefore, the aim of this study was to further promote standardization of the BOD₅/COD ratio parameter on the basis of previous studies (Cossu et al., 2012) to facilitate the establishing of a reference methodology; specifically the targets of the study were to:

• Establish whether contact times of less than 6 h are sufficient for the leaching test.

• Analyze the effects of using different L/S ratios.

• Work with a greater sample of waste (700 g instead of 500 g used in the previous tests) in order to deal with a more representative sample of waste.

• Evaluate the possibility of working directly on waste, without shredding.

• Compare the BOD₅/COD ratio with respirometric index, to confirm the good correlation with traditional stability parameters.