Organic Waste Recycling

180 Organic waste recycling: technology and management

Table 4.13 Characteristics of anaerobic filter process

Advantages Disadvantages

High organic removal capacity
Short HRT
Good adaptation to different wastewater
Application to dilute and high strength
wastewater
No mechanical mixing required
Intensive against load fluctuations
Low area demand

Difficult to start up

Risk for clogging

Restricted to wastewater with low TSS

High TSS and NH 4 -N contents in the

effluent

May require periodic biomass removal

Limited access to reactor interior for

monitoring and inspection of biomass

accumulation

High costs for packing media and support

systems

Because of the physical configuration of the filter, only soluble waste or
wastes with low solid contents including domestic wastewater should be treated
to avoid frequent clogging of the filter.

Up-flow anaerobic sludge blanket (UASB) reactor

This type of reactor, developed by Lettinga et al. (1983) in the Netherlands, is
suitable for the treatment of high-strength organic waste that is low in solid
content (e.g. the agro-industrial wastes mentioned in Chapter 2) with or without
sludge recycle. The digester has three distinct zones: a) a densely packed sludge
layer at the bottom, b) a sludge blanket at the middle, and c) a liquid layer at the
top (Figure 4.16).
Wastewater to be treated enters at the bottom of the reactor and passes
upward through the sludge blanket composed of biologically formed granules.
Treatment occurs as the wastewater comes in contact with the granules. The
gases produced under anaerobic conditions (principally CH 4 and CO 2 ) cause
internal sludge circulation, which helps in the formation and maintenance of the
biological granules. Some gas bubbles, produced within the sludge blanket,
become attached to the biological granules and bring them to the top of the
reactor. The granules that reach the surface hit the bottom of the inverted pan-
like gas/solids separator (degassing baffles), which causes the attached gas
bubbles to be released. The degassed granules typically settle to the sludge
blanket zone, thus creating a long mean cell residence time (șc) and a high solid
concentration in the system. About 80 - 90 percent of the decomposition of the
organic matter takes place in the sludge blanket zone; this occupies about 30
percent of the total volume of the reactor. To keep the sludge blanket in