SLUDGE RECYCLING FOR METHANE FERMENTATION
PROCESSING OF SWINE WASTEWATER
P. Y. Yang, Associate Professor
Y. T. Wong, Graduate Research Assistant
Department of Agricultural Engineering
University of Hawaii at Manoa
Honolulu, Hawaii 96822
A preliminary study for an integrated swine waste management system combining an
anaerobic digestion process and an algal biomass raceway for pollution control, energy
production and utilization has indicated that the system is technically feasible and environmentally sound [ 1 ]. Anaerobic digestion is considered the most important portion of the
system. The design and operational criteria for the anaerobic digestion process should
achieve the goals of maximizing methane yields and production rates, increasing process
stability and decreasing reactor volume. Several innovative digester configurations including
attached film expanded bed process [2], anaerobic fixed film reactor [3], upflow solid
blanket process [4], two-phase digestion process [5], and sludge recycling process [6-8]
have been investigated by using various types of organic wastes.
In general, all these various types of the anaerobic process are capable of achieving
the goals listed above. However, each process configuration has its associated weaknesses
and disadvantages when wastewater characteristics (such as contents of soluble and insoluble organics), pumping requirement, structure of reactor, requirements of operation
and maintenance are involved in design and operation. Among these processes, the sludge
recycling process was selected to observe how the anaerobic digestion can be improved
and stabilized by using swine wastewater containing high organic solids. Additionally,
mathematical models and biological kinetic constants for an anaerobic process with sludge
recycling were developed and evaluated, respectively. Appropriate solid/liquid separation
systems for the anaerobic digestion process with sludge recycling were also investigated.
MATERIALS AND METHODS
Swine wastewater used in the experiment was collected from Oshiro's Hog Farm on
Oahu, Hawaii, and stored at 4"C until prepared for methane fermentation. The program
experiment (semicontinuous flow operation) is summarized as follows:
1. seed: acclimated seed from existing field anaerobic digester (20 m );
2. temperature: 30°C;
3. feeding frequency: twice a week for overall hydraulic retention time (HRT) of 7 and 10
days; three times a week for HRT of 5 days; once a day for HRT of 3 days;
4. reactor volume: 10 liters with mixing device of 150 rpm;
5. ratio of flowrate of recycled sludge to influent flowrate: 0.25;
6. ratio of recycled total volatile solids (TVS) to TVS of effluent: 1.5-2.5 depending on
different HRT applied; and
7. time to steady-state: more than four times the HRT applied.
The operational performance of the methane fermentation process was monitored by
measuring biogas production and TVS concentrations in the influent, effluent and recycled
sludge. Biogas was collected by a special plastic bag and then measured by a wet test gas
meter. The gas component was analyzed by using an indirect chemical method of reaction
of potassium hydroxide and carbon dioxide. The TVS content was measured by following
the recommendation of Standard Methods [9].
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