The MBR Book: Principles and Applications of Membrane

Case studies 257
The company had installed 85 sMBR plants by 2005, of which 58 were dedicated to
treating landfill leachate.
5.4.3.1 Landfill leachate
A landfill site is a large area of ground, normally lined, that is used for tipping and
disposal of waste material. As long as the rainfall is greater than the rate of water
evaporation, then the liquid level (leachate) within the landfill area will tend to rise.
Environmental regulations require that the leachate level be controlled, which
means that excess leachate must be removed and disposed of.
Landfill leachate represents one of the most challenging effluents to treat biologically
(Alvarez et al., 2004). Whilst nitrification is generally readily achievable, with �95%
removal of ammonia reported through the exclusive application of biological techniques,
COD removal is considerably more challenging. Removal efficiency values
range from over 90% to as low as 20% according to leachate characteristics (origin
and, more significantly, age), process type and process operational facets. Treatment
process schemes generally comprise some combination of biological and physical
and/or chemical treatment, with key operational determinants being organic loading
rate and the related HRT. Leachate matrices can be characterised conveniently
with reference to BOD/COD ratio, which normally lies within the range 0.05–0.8
and is generally held to be an indicator of leachate age as well as biodegradability
(Table 5.22). Considerable variability in actual levels of organic material and ammonia
arise, and a low assimilable carbon level combined with a high ammonia concentration
may demand dosing with methanol to ensure an adequate carbon supply.
Leachate not only presents a challenge with respect to biodegradability, but the
mixed liquor generated from leachate is substantially less filterable than that produced
from sewage or other industrial effluents (Fig. 5.40). It is mainly for this reason
that leachate biotreatment by MBRs tends to be a high-energy process, with
specific energy demands ranging between 1.75 and 3.5 kWh/m 3 for standard
sMBRs operating with pumped crossflow.
5.4.3.2 Bilbao
The leachate treatment MBR plant at the landfill in Bilbao (Fig. 5.41) is the largest
such one in the world and has been in operation since 2004. The design inflow to the
plant is 1.8 MLD, though it has actually been treating peak loads of 2.2 MLD. The
process is a classical BIOMEMBRAT ® with a pressurised bioreactor tank operating at
an HRT of 15 h, an SRT of 53 days and an aeration rate of 4000 Nm 3 /h. Following
flow balancing, the leachate is treated in two parallel lines, each consisting of a denitrification
and two nitrification tanks (Fig. 5.42). The sludge from each line is
pumped into two UF plants comprising three streams of 6 modules in series, the permeate
being directly discharged. The membrane plant operates at a mean TMP and
permeate flux of 3 bar and 120 LMH, respectively.
5.4.3.3 Freiburg
The 0.1 MLD Freiburg plant (Fig. 5.43) represents an early example of an airlift MBR
for leachate treatment, having been commissioned in 1999, and is designed for ammonia
and COD removal (Table 5.23). The treatment scheme (Fig. 5.44) comprises flow