The use of quicklime to raise the pH of the sludge from 5.5-6.5 to 10.0-10.5 is known to inhibit the growth of microorganisms. To reduce the likelihood of this occurring, bulking agents and recycled compost are added to the sludge-lime mixture soon after the introduction of the quicklime. The resulting mixture's pH is reduced to 8.5 to 9.0, thus providing an environment conducive to the growth of microorganisms. Refer to Figure 7.15 for the variation in pH as sludge moves through the proposed composting process.

Bulking Agent and Recycled Compost

Composting Pile pH 8.8 -9.0

Finished —Biosolids pH 7.5-8.5

Mixing Device


Figure 7.15 Variation in pH in raw sludge composting.

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