Unit 2.3: Loading Rate

Background and Guiding Questions

Metering feedstocks into a digester at a regular, calculated rate over time is known as the loading rate. Fermenting bacteria are sensitive to the volume and type of feedstocks injected into a digester. Feedstock changes should be done slowly over time so as to not shock the digester which may cause it to stop producing biogas.

Guiding Questions

  • What factors must be considered when loading a digester to grow and maintain optimal fermenting bacteria and methanogen populations?

Loading

The operator must monitor and control digester feeding. The quantity and characteristics of the feedstocks affect the stability and efficiency of the digestion process. In feeding the digester, each of the following must be considered:

  • The concentration of the incoming feedstocks (concentration refers to the amount of solids in a given volume of water);
  • The amount of volatile solids in the incoming feedstock (volatile solids are food for fermenting bacteria and essential for the production of biogas);
  • The amount of inorganic material in the feedstock, such as grit or sand, that does not convert to biogas;
  • The ratio of volatile solids per unit of digester volume (this ratio is used as a loading factor);
  • The hydraulic loading (hydraulic retention time) (adequate hydraulic retention time is necessary to assure microbial growth to convert volatile solids to biogas).

Calculating the Loading Rate

To determine the loading rate of a given digester, some basic information about the digester and the feedstocks coming into the digester are required. The following example illustrates how to calculate the loading rate of a given digester:

Assume 5,000 gallons of manure per day are pumped into a complete mixed digester that is 50 feet in diameter with a manure depth of 20 feet and a cone depth of 5 feet. It is operated at 100 degrees Fahrenheit. An analysis of the manure yielded 6.5 percent total solids and 69 percent volatile solids content. Assume the manure has a specific gravity of 1 (the density of water). What is the solids loading rate?

Calculating the Manure Volume

A. Manure volume in cylinder (tank) = π x radius squared x height

= 3.14 x (25 ft2) x 20 ft

= 39,250 ft3

B. Manure volume in cone = 1/3 x π x radius squared x height or depth

= 1/3 x 3.14 x (25 ft2) x 5 ft

= 1/3 x 6,280 ft3

= 3,271 ft3

C. Total manure volume in digester = volume of cylinder (tank) + volume of cone

= 39,250 ft3 + 3,271 ft3

= 42,521 ft3

Calculating the Loading Rate

A. Pounds of total solids per day = gallons/day x 8.34 lb/gal x % total solids (decimal)

= 5,000 gal/day x 8.34 lb/gal x 0.065

= 2,710 lb/day

B. Pounds of volatile solids per day = lb of total solids/day x % volatile solids (decimal)

= 2,710 lb/day x 0.69

= 1,869 lb/day

C. Loading rate = lb of volatile solids per day ÷ volume of manure in digester

= 1,869 lb/day ÷ 45,521 ft3

= 0.04 lb/day/ft3

Based on the assumptions given above, a loading rate of 0.04 lb volatile solids/day/cubic foot falls within the acceptable loading rate range of 0.02 to 0.37 pounds of volatile solids per cubic foot of digester [4]. This loading rate range can also be expressed as 20 to 370 pounds of volatile solids per day per 1,000 cubic foot.

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