210 Environmental Biotechnology
Case Study 8.1 Integrated Waste Management (Zurich, Switzerland)
As a method of waste management, anaerobic digestion has much to offer, espe-
cially considering its potential for integration into strategies designed to address
wider environmental issues. The Otelfingen Kompogas plant, near Zurich airport,
Switzerland, provides an excellent example.
Constructed in 1997, the half-hectare (1^14 acre) facility processes around 14000
tonnes of source separated biowaste annually, of which some 70% is domestic,
the remainder being generated by commercial activities such as agriculture and
food processing. Incoming waste is shredded and screened to remove any residual
contaminants before being mixed with water to produce a slurry of 28% solids
content for digestion. The bioreactor is of thermophilic plug flow design and has
a retention period of 14–21 days. After processing, the digestate is dewatered by
means of a screw press and the solid component, which is typically well within the
relevant local heavy metal limits, is taken offsite and applied to farmland. Some of
the water element is returned for reuse in the plant, while the excess is a valuable
fertiliser substitute.
The biogas arising is of relatively high quality, being around 60% methane, and
this energy source is exploited in two ways. Firstly, onsite engines are used to
generate heat and electricity, which both covers the needs of the plant and produces
a sizeable surplus for sale. Secondly, the company has invested in technology to
clean up the gas to use as a fuel for its vehicle fleet, contributing a significant cost
saving to its operational budget.
As a final example of biological integration, and of public and private sector
cooperation, 50% funding from the Swiss government enabled the establishment of
aquaculture greenhouses alongside the facility, where attenuated digester effluent is
used to grow a variety of aquatic macrophytes.Case Study 8.2 Co-composting for Reclamation (Colorado, USA)
The main advantages of composting as a means of biological waste treatment lie
in its applicability to a wide range of materials and the potentially useful product
which arises.
The reclamation of the Climax Molybdenum Mine, Leadville, Colorado required
the revegetation of nearly 1600hectares (4000acres) of disturbed land, typically
covered in tailings–the waste rock residue left after ore extraction. This was far from
a straightforward task, since not only does such material provide a very poor footing
for plants in the wind-buffered slopes, but also the growing season is only six or
eight weeks long, the average temperature is slightly above 0◦C and nearly 7metres
(23feet) of snow falls yearly.
The approach employed to overcome these difficulties is a good example of
both the versatility of composting and the ability of biotechnology to provide a
contemporary and integrated solution to more than one environmental problem.
The burgeoning population and rapid growth in the resort towns of Summit County
caused major strain on the existing arrangements for the treatment of sewageContinued on page 211