Recent Advancements in Wastewater Sludge Composting

Many utility providers face growing problems with the disposal
of the wastewater sludges (residuals) that are created as part of the
wastewater treatment process. Other providers are looking to additional methods
for converting the residuals into fertilizer/soil conditioner with a higher
economic and social value. The new technology presented in this paper provides
a composting method to address the disposal and/or use of wastewater residuals.
By maintaining the recommendations presented in this paper, a Class A biosolid
can be produced. This Class A biosolid provides the utility operator the
maximum flexibility for its disposal or use as a fertilizer, soil conditioner,
etc.

Composting of wastewater residual is a bio-thermal aerobic
process that decomposes the organic portion of the residuals. The composting process
reduces the organic material in the residual by approximately 25 percent.
During composting the heat generated by the decomposition of the organic
portion of the residuals reduces the moisture content of the residual,
stabilizes it and renders the residual harmless by transforming it into a
usable biosolid.

Organic Content

In general, the higher the residual’s organic content,
the greater the quantity of heat released during composting. More heat results
in the thermophilic phase (55 to 65° C) being reached earlier in the
composting process. This greater heat release results in more moisture being
evaporated. Raw residual typically contains 60 to 80 percent organic material,
while digested residual contains only 30 to 50 percent organic material. Since
raw residual (from primary clarifiers and secondary clarifiers) contains more
organic material than digested residual, it is reasonable to prepare compost
from dewatered raw residual. By utilizing raw residual the digesters, pipe,
pumps, electrical power, personnel, etc., normally used in the digesting
process can be reduced or eliminated.

The heat generated by composting 1.0 kilogram (kg) of
organic material averages 21 Million Joule (Mjoule). Approximately 4.0 Mjoule
of heat will evaporate 1.0 kg of moisture (taking into account heat losses and
heating of the compost material). Thus, the composting of 1.0 kg of organic
material facilitates the removal of approximately 5.0 kg of moisture from the
residual. [21 Mjoule / (4 Mjoule/kg of water)].

Before composting, it is necessary to dewater the residual.
Dewatering not only reduces the volume of the residual but also decreases the
amount of moisture to be evaporated by the composting process.

Odor

A problem with composting raw residual is the higher intensity
odor that can be released due to the higher percentage of organic material in
the raw residual. While various methods can be used to control the odor, the
authors favor the addition of quick lime (CaO) to change the pH of the
residual. Experiments show that organic material loses its odor when the pH is
raised from the typical 5.5 to 6.5 to a pH of 10.0 to 10.5. In addition to
changing the pH of the residual, the hydration of the quick lime (absorbing
moisture from the residual) causes the quick lime to release heat to the
residual. During the process of hydrating 1 kg of chemically pure quick lime
(100 percent CaO) 1,152 Kilojoules of heat is produced, requiring 320 grams of
moisture/hydration from the residual. This release of heat shortens the time span
of the mesophilic phase (25 to 40° C) and drives the process to the
thermophilic phase (55 to 65° C) quicker, resulting in an overall reduction
in the composting time. If odor continues to be a problem, the simple procedure
of drawing air through the compost piles and discharging the air to a
bio-filter can further reduce the associated odor.

Temperature and Moisture

The temperature increase caused by a predetermined dose of
quick lime may be calculated by the following formula

DT =     (1152
¥ A ¥ Mi) /

                  [(Msi
¥ Csi) + (Mi ¥ Ci)]           (1)

DT         temperature
increase of the residual, °C,

A             quick
lime activity in decimals, typically 0.9

Mi           mass
of quick lime in kilogram (kg)

Msi        mass
of residual in kg

Ci            specific
heat of quick lime =

0.92 Kilojoules / kg°C

Csi          specific
heat of residual in Kilojoules / kg °C that may be

calculated as

 

Csi = 1.8 (1 + 0.85 ¥ Wsi3)     (2)

Wsi         moisture
content (in decimals) of the dewatered residual

 

Raising the residual temperature by 10° C doubles the
speed of the microbiological activity that accomplishes the composting process.

The addition of quick lime absorbs moisture from the
residual, thereby reducing the moisture content of the compost mixture. The
residual moisture content after the addition of quick lime can be calculated
using the following formula:

Wk =    [(Mst
¥ Wsi) – (0.32 ¥ A ¥ Mi)]

                  /
(Msi + Mi)      (3)

Wk         moisture
content (in decimals) of the residual after the addition of quick lime

The lowering of the moisture content of the residual
decreases the volume of the residual. This decreases the amount of quick lime
required to raise the pH to 10.5. Therefore, it is reasonable to dewater the
residual prior to the addition of quick lime.

Composting Mixture

The new compost technology presented in this paper requires
quick lime to be mixed with the dewatered residual prior to adding a bulking
agent (sawdust, peat, woodchips, bark, hydrolyzed liquin, etc.) and recycled
compost. A technological schematic of raw wastewater residual composting
process is shown in Figure 1. Once the quick lime and residual are thoroughly
mixed, the bulking agent and a portion of recycled compost are added and mixed.
This mixture then is formed into piles and allowed to compost until a
temperature of 55 to 65° C has been maintained for 3 to 11 days. The piles
often are covered with a layer of bulking agent or recycled compost to protect
the pile from heat loss as well as avoid attracting flies, mosquitoes and other
undesirable insects. A list of recommendations for the mixture is shown in
Table 1.

Compost Process Control

Experiments show that the type and population of
microorganisms varies during the composting process. Therefore, it is critical
to control the composting environment so that the microorganisms can flourish.
The composting environment parameters include the compost pile temperature,
moisture content of the compost, oxygen and carbon dioxide levels in the
compost pile and the availability of nutrients including carbon, nitrogen,
phosphorus and potassium for the microorganisms. These parameters must be
monitored as they affect the vitality of the microorganisms.

The temperature in the compost pile most directly affects
the type of microorganisms and their functions. The type of microorganisms
change as the compost pile temperature increases from its initial temperature
to the mesophilic phase to the thermophilic phases and to the slow decrease in
temperature following to completion of the composting process. Experiments show
that the thermophilic phase must be maintained for 3 to 11 days to produce a
Class A biosolid. It is during the thermophilic phase that most pathogens are
destroyed. As the type of microorganisms changes in relation to the compost
pile temperature, so does their requirements for moisture and oxygen. The
moisture content of the compost, oxygen and carbon dioxide levels in the
compost pile and compost pile temperature is closely related to one another. A
change in one directly affects the others.

Oxygen is supplied to the compost pile by the introduction
of air. The rate of air supplied depends on the moisture content of the compost
pile. The higher the moisture content the higher rate of air is required. A
minimum oxygen level must be maintained while carbon dioxide levels must not be
allowed to exceed a maximum level. As air is supplied, the porosity of the pile
increases. That process leads to increased evaporation and a resultant decrease
in the moisture content of the pile. Supplying air also can lead to heat losses
that result in a temperature reduction within the compost pile. This
temperature reduction results in a lower rate of microorganism functions.
Therefore, the oxygen and carbon dioxide levels and the amount of air supplied
must be monitored and controlled. Experiments indicate the rate of air supplied
is approximately 15 to 20 cubic meters per hour for each ton of organic
material being composted. Monitors and controllers should be used to
automatically supply air to the pile when the carbon dioxide level within the
pile reaches 8 percent.

Data obtained from raw residual composting experiments have
produced the

Example

A Wastewater Treatment Plant (WWTP) with a design capacity
of 40 million gallons per day (mgd) generates approximately 700 ton/day of
thickened mixture of primary/ thickened activated residual with moisture
content of 97 percent or 21 ton/day of dry solids (3 percent solids). The
WWTP’s dewatering system of centrifuges with polymer feed dewaters the
residual to a moisture content of 80 percent (20 percent solids). This reduces
the mass of residual to 105 ton/day. The composting process utilizes the
addition of quick lime and the addition of sawdust as a bulking agent. (Note:
The utilization of polymer by dewatered raw residual is almost two times less
than by dewatered anaerobic digested residual.) The addition of 2.1 ton/day of
quick lime (105 ton/day ¥ 2 percent) increases the pH to 10.5, removes the
odor from the residual and increases the temperature of residual by

DT =     (1,152
¥ A ¥ Mi) / [(Msi ¥ Csi)

                  +
(Mi ¥ Ci)]      (4)

 

Csi = 1.8 (1 + 0.85 ¥ Wsi3)      (5)

DT =     (1,152
¥ 0.9 ¥ 2,100) / [(105,000

                  ¥
{1.8 [1 + (0.85 ¥ 0.83)]})

                  +
(2,100 ¥ 0. 92)]

DT = 2,177,280 / 273,184.8

 

DT = 8.0° C

Duration of the composting period and thermophilic phase
depends on process performance, quantity and composition of the compost mass
(moisture content, organic and chemical content of the residual, type of
bulking agent, viability of the recycled compost, etc.) and can last from
several days to several weeks. For example, by using biodegraded wood chips
and/or recycled compost, the temperature in the compost pile increases at a
faster rate since these materials already are in a state of biodegradation.
Quick lime added to the residual shortens the composting process by increasing
the starting temperature through a chemical reaction. An example of temperature
versus duration is the composting of raw wastewater residuals with quick lime
compared to composting of digested wastewater residuals without quick lime as
shown in Figure 2.

The residual moisture content after the quick lime addition
is

Wk =    [(Mst
¥ Wsi) – (0.32 ¥ A ¥ Mi)] /

                  (Msi
+ Mi)          (6)

Wk =    [(105,000
¥ 0.8) – (0.32 ¥ 0.9

                  ¥
2,100)] / (105,000 + 2,100)

                  =
0.78 or 78 percent.

The quantity of sawdust added is 105 ton/day (105 ton/day
¥ 1.0) and recycled compost added is 21 ton/day (105 ton/day ¥
0.2).                         

The technological scheme of raw wastewater residual
dewatering and composting is shown in Figure 3.

It takes several days to reach the thermophilic temperature
with the addition of quick lime. Maintaining that temperature for 10 to 11 days
(Figure 2) provides the highest level of pathogen reduction/vector control and
produces a compost meeting the 40CFR Part 503 Biosolids Rule. By comparison, an
aerated static-pile system requires longer composting times and more
operational processes. These two issues result in a large area being required
to store the composting materials.

Conclusion

This new technology offers a cost-effective method of raw
residual composting that reduces or eliminates the need for digesters and all
associated expenses. This technology includes the addition of quick lime and
bulking agents to the raw residual and the recycling of compost to aid in the
composting process. This technology facilitates the elimination of odor,
increases the temperature at the beginning of the composting process and
reduces the duration of the composting process.

Izrail S. Turovskiy, D. Sc. is a wastewater and sludge treatment consultant in Jacksonville, Fla. Izrail has more than 45 years experience in the field including a time when he was a department head in the All-Union Research Institute of Water Supply, Sewage Systems and Hydrotechnical Structures located in Moscow, Russia. Jeffrey D. Westbrook, P.E., is a senior environmental project manager with Jacobs Civil, Inc., Jacksonville, Fla. Jeffrey has more than 20 years experience in potable water, wastewater and reclaimed water utilities/treatment facilities.

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