190 COMPOSTING
in use or under construction. The largest, a tunnel reactor
in Baltimore, Maryland, has a throughput of 2,500 tons per
week. Another facility, in Sevier County, Tennessee, using
a continuous flow rotating tunnel digester, processes 1,100
tons of MSW per week; in St. Cloud, Minnesota, a digester
with an agitated bed, processes 250 tons of MSW per week;
in Easthampton, New York, a recently opened facility uses
an in-vessel, agitated-bed digester to process 180 tons of
source-separated and mixed MSW per week. 16,17
Continuous flow operations have a number of common
unit processes: feedstock preparation, storage, biochemical
activation/stabilization and curing. After preparation (see
Fundamentals: Preparation, above), the feedstock is trans-
ported to a storage vessel via mechanized conveyors; from there
it is fed into the biological digester on a more or less continu-
ous basis. The biochemical reactions that take place in-vessel
are the same as those in batch operations (see Fundamentals):
a succession of microorganisms attack the substrate, tem-
peratures rise to 55–65C, and the initial material undergoes
chemical and physical transformation. At some facilities, the
in-coming waste material is seeded with microorganisms
from compost produced on site; in others, waste water sludges
or animal manures provide the necessary microbial culture to
maintain peak-activity within the digester. Nutritional supple-
ments may also be added to accelerate the process or to supply
a nutrient that can be used in the finished product.
The amount of time the feedstock remains in the digester
depends on the design of the system and the incoming flow of
the waste stream. For example, the Baltimore tunnel system
(65 × 18d.) “digests” material for 12–20 days; at the
Sevier Country facility, the rotating digester (185 × 12d .)
stabilizes the MSW feedstock in 3 days; in Easthampton, the
agitated-bed system (6 × 6× 250 troughs) stabilizes waste
materials in 21 days.^14 In continuous operations, the accel-
erated rates of microbial attack characteristic of in-vessel
systems, require that moisture, temperature and oxygen be
monitored carefully and maintained at as close to optimal
levels as possible, and most process controls are now run by
computer. It should also be noted that in the higher tempera-
tures sustained in the digester, thermophilic microorganisms
are the predominant population.Curing and Final ProcessingIn continuous flow operations, biological stabilization is nor-
mally followed by some type of aerated curing. The curing
lets the compost mature to a point where significant micro-
bial activity has subsided. During this stage, phytotoxicity
(characteristic of immature compost) disappears and avail-
able nitrogen (mostly nitrate-N) rises to higher levels. At
the Baltimore facility, stabilized material is cured in static,
negatively-aerated piles, while at the St. Cloud facility, sta-
bilized material matures in mechanically turned, positively-
aerated, 200 trenches; at the Easthampton plant, negatively
aerated windrows are used.^17
After a period of 1–3 months of curing, ambient tem-
peratures are regained, and the finished compost is ready for
application. Using a mechanized rotary drum screens, finishedcompost is screened to various particle sizes for use in agri-
cultural, horticultural or landscape applications. While most
field agricultural application does not require final processing,
screening compost to a 3/4 inch size is common for landscap-
ing; while a 1/4 inch screened compost product is more likely
to be bagged and marketed at gardening centers. Generally,
high in available nitrogen, low in ammonia and heavy metal
concentrations, top-grade compost intended for use as a soil
amendment, conditioner or fertilizer can be customized during
the latter stages of the curing process to suit the users’ needs.
The parameters most commonly balanced for specific use
are the NPK ratio (nitrogen, phosphorous, potassium) and solu-
ble salt content (i.e., the conductivity of the finished product).UTILIZATIONThe recent growth of MSW composting facilities in the U.S.
can be attributed to the necessity, in communities all over the
country, to minimize the volume of waste being landfilled and
incinerated. As composting takes a larger and larger bite out
of the solid waste stream, and as a wider spectrum materials
are composted, greater quantities of compost are being pro-
duced. To ensure product marketability and continued industry
growth, solid waste managers and facility operators will now
have to place greater emphasis on quality control, customer
satisfaction and product utilization. 18,19 They will have to
approach composting as a manufacturing process: The biode-
gradability of the incoming material must be well understood;
systems and processes designed to maximize consistency and
maturity; markets developed for a range of compost applica-
tions based on user’s needs and product benefits (e.g., water
conservation, fertilizer savings, increased yield/acre, stronger
plants, pesticide savings, disease suppression^20 bioremedia-
tion of hazardous sites and land reclamation).QualityCurrently, compost producers, Departments of Agriculture
in a number of states and the Composting Council (a non-
profit industry association) are working together to develop
compost quality standards and application (i.e., product
use) guidelines. These standards will draw on the U.S.
Environmental Protection Agency’s Part 503 biosolids rule
which established health, safety and environmental stan-
dards for composts from waste water sludges. When imple-
mented these standards will enable operators to improve
process design; they will provide end users with quantita-
tive and qualitative data on specific parameters of a compost
product and in general further awareness of the interrelation-
ship of the composting process and the quality of the end
product.^21 Quantitative standards will include: pH, soluble
salt content, water holding capacity, bulk density, moisture
content, organic matter content and particle size. Qualitative
and unspecified standards will include concentrations of
trace elements/heavy metals and compost stability (see
Fundamentals: Biological Stabilization ) and maturity.^19C003_003_r03.indd 190C003_003_r03.indd 190 11/18/2005 1:00:20 PM11/18/2005 1:00:20 P