Environmental Engineering Reference
In-Depth Information
12.8
By-product accumulation in membrane purification processes of
digestate.
Membrane filtration
When applying membrane technology to the liquid fraction of digestate,
enhanced solids removal has to be performed first. This is a crucial point in
membrane purification processes, alongside of membrane fouling. Decanter
centrifuges are usually used in the first solid-liquid separation step and
precipitating agents are often added for increased solids removal. The next
step is ultrafiltration and, finally, reverse osmosis is used for removal of
ammonia and COD (chemical oxygen demand). Normally, three steps of
reverse osmosis are needed to reach discharge levels for ammonia. A
drawback of such membrane purification processes is that only a limited
amount of the digestate will be purified water; about 50% of the digestate is
accumulated as by-products (see Fig. 12.8). As an alternative, the last reverse
osmosis step can be replaced by ion exchange in which the solid fraction,
ultrafiltration retentate and reverse osmosis concentrate are accumulated. In
order to reduce amounts, the ultrafiltration retentate is often recycled into
the biogas plant and/or the solid-liquid separation step. Membrane
purification is quite expensive and requires a considerable amount of energy.
Aerobic treatment
In an ideal situation, the liquid fraction of the digestate can be treated
together with another wastewater (rich in carbon) in an aerobic wastewater
treatment plant. If this is not the case, an external carbon source will have to
be added because, due to the high ammonia content, the bacteria need
excess carbon in order to carry out the denitrification step. Apart from the
nitrogen problem, residual COD and the color of the treated effluent
(turbidity) make it difficult to meet required discharge levels. In addition,
aerobic wastewater treatment means high investment and operation costs.
Search WWH ::
Custom Search