A troublesome byproduct of intensive livestock production is manure.
The only sensible solution is recycling of manure by recovering purified
water which can be used for irrigation and the production of dry, manageable
fertilizers.
After different combinations of pretreatment operations, the
remaining liquid part of the manure is treated by thermal separation technology
including evaporation and drying.
In the evaporation plant, a major part of the water is evaporated from the
manure in multiple-effect vacuum operation, which has an advantageous influence
on the energy and investment costs of the downstream dryer.
If the manure is evaporated in conventional falling film evaporator types,
it may tend to form considerable incrustation on the heating surfaces. In order
to prevent this incrustation from forming on the heating surfaces of the
evaporation plant, the calandrias are designed as forced circulation or even as
fluidized bed heat exchangers if the number of cleaning periods are to be
reduced to a minimum. These heat exchangers, which have been in particular
developed for medium forming incrustation, are permanently cleaned during
operation by means of a circulating fluidized bed of small stainless steel
particles.
In the evaporation plant the liquid is concentrated to 20 - 40 % DS
depending on the type of pretreatment. The concentrate from the evaporator and
the separated solids from mechanical treatment are mixed and dried in different
types of dryers.
From the final solids, granulate or powder which can be
transformed into fertilizer pellets is produced. The water evaporated can be
used for irrigation.
An alternative, which promotes the construction of this type of plant is the
combination of manure treatment plants with cogeneration plants. The main
feature of this combination is the power generation. The waste heat produced in
the form of hot motor cooling water of approx. 95° C can be used for heating
the evaporation plant. The hot waste gas also produced is used for heating the
dryer. As a consequence, the energy costs are reduced to a negligible
minimum.