CCC Technical Corner: Bulk density1

Technical Corner

By Craig Coker, Staff Writer

(Editor’s Note: this is the first in a series of articles on field exercises that can be used for compost operator training. These field exercises will be used in the Council’s Native American Compost Operator Training Course in Cherokee, NC this summer).

Bulk Density Determination
and Conversion

The bulk density (BD), determined by bucket drop method, is the ratio of the mass of the materials (wet or dry) to its volume. BD is used to convert compost recipes from a weight (mass) basis to a volume basis for field mixing. The bulk density figures will be used to convert the recipe from weight to volume.

Equipment:

Tape measure or 12-inch ruler & Black marking pen;

Water

5-gallon plastic pail (two pails may be needed in the field, one for the sample and one for water). Five gallons should measure to the brim of the pail with no additional holding capacity.

Scale: 0-50 pounds, fish scale type preferred or platform type

Procedure:

1. Make a series of dash-markets around the inside circumference of the bucket 1/3 and 2/3 from the rim of the pail (the 5-gal. fill-level). This marks the 1/3 and 2/3 fill levels.

2. Obtain a weight of the clean, dry empty bucket and record the weight.

3. Gather a sample of the material to be analyzed.

4. Fill the bucket 1/3 full with the material.

5. Pick-up the pail by the handle and drop the bucket containing the sample onto a firm flat surface from a height of 6-inches ten times, being careful to guide it with the handle so it stays upright and doesn’t tip.

6. Now add additional feedstock and fill to the two-thirds full level.

7. Repeat the procedure (#5) a second time, dropping the pail ten times from 6-inches.

8. Add material to the pail to fill it to the top,

9. Repeat the dropping procedure (#5) a third time. After the third time, fill the bucket to the rim, but do not repeat the dropping procedure (#5).

10. Weigh the pail and its contents on as accurate a scale as is available.

11. Record the weight of the 5-gallon bucket filled to the top with sample material.

12. Subtract the weight of the empty bucket (2.2 lbs) to obtain the weight of the sample. Example: The total weight of a 5-gallon pail filled to the top with sample material may be about 28.5 pounds; subtract the weight of the empty pail (2.2 lbs) to obtain the weight of the sample contained in the pail and record, i.e. 28.5 – 2.2 = 26.3 pounds

13. Divide the weight of the material by 0.025 (the volume of the bucket). Record result on space for Bulk Density (BD).

Feedstock _______________________

1. Weight of 5-gallon pail filled with feedstock _________ lbs.

2. Subtract the weight of the empty 5-gal. pail (-)_________ lbs.

(=)_________ lbs.

3. Divide by 0.025 (volume of bucket) _______ = Bulk density (D1) (lbs/cy)

Feedstock _______________________

Weight of 5-gallon pail filled with feedstock __________ lbs.

Subtract the weight of the empty 5-gal. pail (-)__________ lbs.

(=)__________ lbs.

Divide by 0.025 (volume of bucket) _________ = Bulk density (D2) (lbs/cy)

Convert Recipe from Weight to Volume

To be practical in a typical composting situation, the quantities of feedstocks (calculated in Step 1) need to be converted to volume, i.e. for a front end loader bucket load, how much of each material should I use in the mix?

Use the formula V = Q/D (Volume = quantity divided by density).

The amount of Q1 & Q2 was calculated with the spreadsheet in class.

V1 = Q1/D1 = _________________ cu. yds.; V2 = Q2/D2 = __________________ cu. yds.

This will result in tiny volumes for both materials. To make this amount useable in terms of buckets, divide V2/V1 = __________________. This result is V2 (expressed in bucket loads) that needs to be mixed with each full bucket of V1. If the answer (V2) is a percent that is not easily expressed as a fraction i.e., ¼, ½, etc. round to the nearest useful measure for a bucket such as "a little over a half, a little under a half, etc."