Compost / Soil Testing

Respirometric Measurement of Oxygen Uptake or Biogas Production in Soil and Compost Samples

The PF-8000 respirometer system is ideally suited for measuring oxygen uptake rates or biogas production in soil and compost reactions.   Two test configurations are available:  a static method in which a sample of soil or compost is placed in vessel with resulting measure of oxygen uptake for aerobic tests (for example ISO 17556 protocol) and biogas production for anaerobic tests (for example ASTM D5511).   For aerobic tests, carbon dioxide is absorbed in a vial of caustic material suspended within the headspace of the reaction vessel.  Mixing typically is not provided.  Results of such tests usually are expressed in milligrams of oxygen uptake or milliliters of gas production per gram of soil or compost.

Reaction Vessel

Schematic diagram of static compost/soil testing apparatus.

Example data is shown below for a sample of municipal solid waste which was tested for stability before and after composting using the German AT4 method (German Ordinance on Environmentally Compatible  Storage of Waste from Human Settlements and on Biological Waste Treatment Facilities, 2001 ( nung.pdf). Samples were first ground to particle size less than 1/8 in (3 mm).  Twenty grams of dry sample plus 20 g of nutrient/mineral/buffer water were placed into respective respirometer vessels.   The purpose of the perforated SS sleeve was to spread the sample out to maximize the opportunity for oxygen diffusion into the sample.  No mixing was provided.  Oxygen uptake was measured using a PF-8000 aerobic/anaerobic respirometer system.   Figure A shows cumulative oxygen uptake over ten days of testing expressed as milligrams of oxygen per gram of dry solids for both a raw sample and a composted sample.   Figure B shows respective oxygen uptake rates.  The OUR data show one oxygen uptake zone associated with easily compostable organics (blue highlighted zone) and one zone associated with slowly degradable organics (green highlighted zone).   Very little oxygen uptake was associated with the after-composting sample, thereby demonstrating the effectiveness of the composting operation.

A second reactor configuration is to use a dual-column apparatus as illustrated below.  This approach measures in-situ rates of oxygen uptake in soil or compost samples – as may occur in the upper layers of soil under natural atmospheric conditions or in mixed compost reactors.  In this case, the headspace gas is circulated throughout the column using an internal fan so that oxygen diffusion into the sample matrix is not a limiting factor. 

Dual-column reactor

Schematic diagram showing use of dual-column reactor for measuring oxygen uptake in soil or compost samples.

Soil/Compost columns in a water bath.

Methane Production during Anaerobic Composting of Municipal Solid Waste

Interest has been shown in converting municipal solids waste (garbage) to methane gas in an anaerobic environment (for example using ASTM D5511 protocol).    In one test, the researcher used two different sources of municipal solid waste and two sources of microorganism inoculum.   The test setup consisted of adding the solid waste and inoculum mixture to the soil/compost column configuration as shown above.  The columns were placed in a water bath for temperature control and incubated at 55oC.  

The test results, shown below, indicate that Inoculum 1 started producing methane gas almost immediately with both solid waste samples.   Inoculum 2 showed a lag followed by recovery and eventually produced more methane than the columns receiving Inoculum 1.   The longer lag with Inoculum 2 was caused by the acidic conditions of the inoculum source and the higher methane production was attributed to the larger amounts of organic acids that were produced in this sample.

Aerobic Treatment of Oily Soil

An oily soil from a refinery site contained 4.7% hydrocarbons by weight.   The total COD of the soil was 55,000 mg/L.   Tests were conducted to determine if Tween-80, a surfactant, enhanced the biodegradation of the oily contaminants in the soil.  Samples of the oily soil were added to individual reactors of an aerobic respirometer system.  Tween-80 was added on one set of samples at a T-80:soil ratio (w/w) of 1:10.  The tests were conducted for 52 days without interruption.

The respirometer tests showed that Tween-80 enhanced the biodegradation of the organic materials in the soil sample.   Without T-80, the 52-day oxygen uptake represented 30% of the COD while with T-80, the 52-day oxygen uptake represented 67% of the COD as shown in the figure below  (All data have been corrected for dilution and for the contribution of seed organisms to the oxygen uptake).

Oxygen uptake of oily soil as measured by a respirometer using slurry reactors.

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