Owing to the nature of the particles and their fine grain size, the material has a very low hydraulic permeability, very high water content and very low shear strength.
Frequently, sewage treatment processes produce a sludge cake which has a solids content in the range of 10 – 20%. This material is difficult to handle, being ‘too thin to shovel and too thick to pump’.
Electrokinetic has developed applications of EKG to dewater
sewage sludge cake by electroosmosis. Electroosmosis occurs in sewage sludges
because the solid materials have a negative surface charge similar to those
seen in clays and silts.
Sewage lagoons
Historically sewage sludge cake has been deposited in purpose made holding
lagoons. In arid parts of the globe, the material in such lagoons slowly dries
out. However in more temperate latitudes, rainfall prevents the drying out
of the material. In many cases the material can remain in a lagoon for decades
with little or no reduction in moisture content or increase in solids content.
These lagoons are often sited next to rivers and on the edges of urban areas.
This means that lagoons can at the same time pose an environmental threat
but also present a development opportunity.
Therefore there are economic and environmental drivers to clean these areas up, but the peculiar nature of sludge cake makes this a significant technical challenge. The alternatives are:
- Dispose of the material to landfill. This has both environmental and economic constraints
- Cover it with a strong membrane. This encapsulation process should reduce the leaching in time but prevents the ground from being developed
- Mix a stabilising material into the sludge to strengthen it. This is a very slow, expensive and hazardous process
The EKG approach is to dewater the sludge in situ either as a precursor to disposal or a first stage of ground improvement for development.
Laboratory scale, single polarity electroosmotic consolidation was carried out on a sample of sewage sludge from a sewage works in northern England. The results showed:
- an overall volume reduction of 57%
- an increase in % dry solids from 19% to a maximum of 42%
- an increase in shear strength from approximately 2kPa to a maximum of 29kPa
Subsequent tests on sewage sludges indicate that although these materials
vary appreciably in their composition, they invariably support the application
of electroosmosis.
Electrokinetic Ltd. is working with major water companies
to apply EKG technology to some persistent problems associated with dewatering
and stabilising sewage sludges.
Waste processing
In addition to the dewatering of sewage lagoons, there is increasing interest
in the use of electrokinetics as a dewatering method in the active processing
of sewage sludge and mine tailings. Trials at Electrokinetic
have shown that EKG can produce important gains in dry solids contents when
compared to standard hydraulic techniques, see graph below.
Humic sludge
Use of EKG to increase rate of dewatering of humic sewage sludge over that achieved by hydraulic means alone (Control).
