Thermophilic Decomposition:

 

Thermophilic Decomposition:

It is a composting method which produces complete aerobic digestion and pasteurization of animal and other solid organic waste materials is effected in an open top, elongate, composting vessel, for producing an odor free, pasteurized, organic fertilizer, mulch or animal feed supplement.

Introduction: rotating-tank" type of in-vessel composter to rapidly decompose and stabilize institutional food service (cafeteria-type) food residuals have experienced delays in attainment of thermophilic temperatures.  On occasion, food residuals loaded into the in-vessel composter soon after collection (within 1-2 hours) would reach thermophilic temperatures within 24-36 hours.  However, when loading was delayed for 3-6 hours after collection, a fermentation process apparently began which produced fermentation odors (resembling a typical "trash-truck" smell) within 6-12 hours.  Simultaneously, compost pH would rapidly decline to near 3.0 and remain at or near that level for several days, during which time temperatures would remain in the mesophilic range.

In order to reduce the retention time inside the in-vessel composter and make most efficient use of the technology, reducing or preventing the lag time in achieving thermophilic decomposition is desirable.

How to make composter:

 

The composters were constructed of open-ended metal tanks measuring 3 feet in diameter and 6 feet long resting in a horizontal position on a set of 4 casters.  The tanks were rotated at the rate of 4 revolutions per hour, 24 hours per day, by a 0.5 hp motor using a rachet-drive mechanism.  The ends of the tanks were enclosed by plywood doors with a 0.5 inch perimeter gap for ambient air exchange.  Each tank was fitted with a center partition creating two compartments measuring 3 feet in length and 3 feet in diameter.  Each chamber would hold 0.5 cubic yards of material when filled to approximately two-thirds capacity, allowing headspace in each chamber for air exchange

Hydrated lime was added at the time of compost loading at the rates of 0, 5, 10 or 15 lbs. per 0.5 cubic yard batch.  Treatments were not replicated.  Temperature and pH were measured and recorded daily for 14 days.

RESULTS

The pH of compost receiving no lime was approaching 3.0 by 24 hours after loading (day 1).  The pH varied between 3.0 and 5.0 for the next 10 days, at which time a consistent increase began, reaching 8.2 on day 14.  Compost temperature ranged slightly below 120 degrees F through day 10.  Coinciding with the pH increase, compost temperatures rose to 144 degrees F by day 14.

The compost receiving 5 lbs. of lime started at pH 7.5, but gradually declined to a low of 4.3 on day 7.  However, by day 10, pH had increased 8.0.  As with the no-lime treatment, compost temperatures tended to parallel changes in pH.

The addition of 10 lbs. of lime increased the initial pH to 11.7.  However, the pH declined to 7.5 on day 4, which was the same day that temperatures reached the thermophilic range.  The pH ranged between 7.3 and 8.9 for the remainder of the project.

Adding 15 lbs. of lime increased the initial pH to 12.3.  The pH dropped below 8.0 and compost temperature reached the thermophilic range on day 7.

SUMMARY

The no-lime treatment required 12 days to reach thermophilic temperatures whereas 5 lbs. of lime allowed thermophilic temperatures to be attained by day 2.  However, the temperatures cooled into the mesophilic range as the pH dropped to approximately 5.5 and did not re-enter the thermophilic range until the pH increased.  The 10 lb. lime treatment required 4 days and the 15 lb. treatment required 7 days to reach thermophilic temperatures.

 

CONCLUSIONS

During the first few days of composting, a close relationship appears to exist between compost pH and temperature.  Addition of lime can reduce the time required to reach thermophilic composting temperatures from 12 days down to as low as 2 days.  With food residuals of the type used in this study, the optimum rate of hydrated lime addition appears to be between 5 and 10 lbs. per 0.5 cubic yard quantity.  Additional work is needed to study changes occurring between the time of initial food collection and initiation of composting,  appropriate rates of lime addition, and the need for lime when using other types of food residuals.