Procedures and Performance

At an operation in Lufkin, Texas, thickened (3.5 to 4% solids) primary and waste-activated sludge are sprayed at a rate of 0.05 lb/ft2-d (0.24 kg/m2-d) dry solids over beds containing worms and sawdust. The latter acts as a bulking agent and absorbs some of the liquid, assisting in maintaining aerobic conditions. An additional layer of sawdust, 1 to 2 in. (2.5 to 5 cm) thick, is added to the bed after about 2 months. The original sawdust depth was about 8 in. (20 cm) when the beds were placed in operation. The mixture of earthworms, castings, and sawdust is removed every 6 to 12 months. A small front-end loader is driven into the bed to move the material into windrows. A food source is spread adjacent to the windrows, and within 2 days essentially all the worms have migrated to the new material. The concentrated worms are collected and used to inoculate a new bed. The castings and sawdust residue are removed, and the bed is prepared for the next cycles.

Human pathogen reduction in a field experiment with vermiculture (vermi-composting) was found to reduce fecal coliforms, Salmonella spp., enteric viruses, and helminth ova more effectively than composting (Eastman et al., 2001). The ratio of earthworms (Eisenia foetida) to biosolids was 1:1.5 wet weight. After 144 hr, fecal coliforms showed a 6.4-log reduction, while a control experiment showed only a 1.6-log reduction. Salmonella spp. reduction was 8.6 log, and the control reduction was 4.9 log. Enteric viruses were reduced by 4.6 log as compared to 1.8 log reduction in the control. Helminth ova reduction was 1.9 log vs. 0.6 log in the control.

Example 9.4

Determine the bed area required to utilize vermistabilization for a municipal wastewater treatment facility serving 10,000 to 15,000 people. Compare the advantages of liquid vs. thickened sludge.

Solution:

1. Assuming an activated sludge system or the equivalent, the daily sludge production will be about 1 mt dry solids per day. If the sludge contains about 65% volatile solids (see Table 9.2), the Cornell loading rate of 1 kg /m2-wk is equal to 1.54 kg/m2-wk of total solids. Assume downtime of 2 wk per year for bed cleaning and general maintenance. The Lufkin, Texas, loading rate for thickened sludge is equal to 1.78 kg/m2-wk of total solids.

2. Calculate the bed area for liquid (1% solids or less) and for thickened (3 to 4% solids) sludges.

For liquid sludge:

Bed area = (1000 kg/d)(365 d/yr)/(1.54 kg/m2-wk)(50 wk) = 4740 m2

For thickened sludge:

Bed area = (1000 kg/d)(365 d/yr)/(1.78 kg(m2-wk)(50 wk) = 4101 m2

3. A cost analysis is required to identify the most cost-effective alternative. The smaller bed area for the second case is offset by the added costs required to build and operate a sludge thickener.

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