Wastewater Calculations - Mathematics and Statistics for the Environment - page 741

Key Point: When more than one metal is present, the calculations must be performed for each
metal. The site life would then be the lowest value generated by these calculations.

24.14.5.2 Biosolids to Compost
The purpose of composting biosolids is to stabilize the organic matter, reduce volume, eliminate
pathogenic organisms, and produce a product that can be used as a soil amendment or conditioner.
Composting is a biological process. In a composting operation, dewatered solids are usually mixed
with a bulking agent (e.g., hardwood chips) and stored until biological stabilization occurs. The
composting mixture is ventilated during storage to provide sufficient oxygen for oxidation and to
prevent odors. After stabilization of the solids, they are separated from the bulking agent. The
composted solids are then stored for curing and are applied to farm lands or other beneficial uses.
Expected performance of the composting operation for both percent volatile matter reduction and
percent moisture reduction ranges from 40 to 60%.
Performance factors related to biosolids composting include moisture content, temperature,
pH, nutrient availability, and aeration. The biosolids must contain sufficient moisture to sup-
port the biological activity. If the moisture level is too low (40% less), biological activity will be
reduced or stopped. At the same time, if the moisture level exceeds approximately 60%, it will
prevent sufficient airflow through the mixture. The composting process operates best when the
temperature is maintained within an operating range of 130 to 140°F; biological activities provide
enough heat to increase the temperature well above this range. Forced air ventilation or mixing
is used to remove heat and maintain the desired operating temperature range. The temperature
of the composting solids, when maintained at the required levels, will be sufficient to remove
pathogenic organisms. The influent pH can affect the performance of the process if extreme (less
than 6.0 or greater than 11.0). The pH during composting may have some impact on the biological
activity but does not appear to be a major factor. Composted biosolids generally have a pH in the
range of 6.8 to 7.5. The critical nutrient in the composting process is nitrogen. The process works
best when the ratio of nitrogen to carbon is in the range of 26 to 30 carbon to one nitrogen. Above
this ratio, composting is slowed. Below this ratio, the nitrogen content of the final product may
be less attractive as compost. Aeration is essential to provide oxygen to the process and to control
the temperature. In forced air processes, some means of odor control should be included in the
design of the aeration system.
Pertinent composting process control calculations include determination of percent of moisture
of compost mixture and compost site capacity.

24.14.5.2.1 Blending Dewatered Biosolids with Composted Biosolids
Blending composted material with dewatered biosolids is similar to blending two different percent
solids biosolids. The percent solids (or percent moisture) content of the mixture will always fall
somewhere between the percent solids (or percent moisture) concentrations of the two materials
being mixed. Equation 24.168 is used to determine percent moisture of mixture:

( )

× (

)

+

Biosolids(lb/day)%Moi × st. 100 Compost(lb/day)%Moist. 100

%Moisture
of mixture

(24.168)

=

Biosolids(lb/day)Compost(lb/day) +

■ EXAMPLE 24.136
Problem: If 5000 lb/day dewatered biosolids is mixed with 2000 lb/day compost, what is the percent
moisture of the blend? The dewatered biosolids has a solids content of 25% (75% moisture) and the
compost has a 30% moisture content.