The MBR Book: Principles and Applications of Membrane

236 The MBR Book
Table 5.12 Membrane characteristics and maintenance, Perthes
en Gatinais plant
Parameter Value
TMP range 0.2–0.5 bar
Backwash frequency and duration Every 10 min for 45 s
CIP Once a week with hypochlorite
External chemical cleaning Hypochlorite/acid soak 1–4 times a year
Coarse bubble aeration is applied at a rate of 1 m 3 /h per m 2 membrane area. The
permeability is routinely between 70 and 120 LMH/bar over the range of TMP indicated
in Table 5.12. Regular flushing, around once every 10 min, together with the
CIP procedures outlined (Table 5.12), enable the required steady-state flux rates to
be maintained. External recovery cleaning with sodium hypoclorite is conducted by
taking the membrane out of the aeration tank and transferring it to an adjacent
cleaning tank where both cleaning chemical reagents (acid and sodium hypochlorite)
and cleaning durations can be employed more flexibly. Cassettes in the aeration
tank can be backflushed separately, and the backflushing process demands
around 25% of the product permeate.
Full nutrient removal is achieved by the plant. Phosphate removal is achieved by
dosing ferric chloride into the effluent, the phosphate being removed with the excess
sludge. The resulting excess sludge production, including the solids created by chemical
dosing, is around 0.5 kg TSS/kg COD.
The dewatering unit consists of a sludge thickener combined with a belt press. The
total sludge production is �270 kg/day. To achieve a satisfactory thickness of 16%
after the belt press, a polymer dose of 6 kg/ton DS is required. After dewatering, the
solids content of the sludge product is increased and the sludge is further stabilised
with lime at a rate of 0.52 kg CaO/kg. The end product (Fig. 5.22) has a DS concentration
25 wt% and is distributed to local farmers and disposed to land.
5.3.1.2 Brescia sewage treatment works
Background At the existing conventional plant at Brescia, the Verziano WWTP had
three trains, each consisting of primary clarification, biological oxidation, secondary
clarification and final chlorination (Fig. 5.24). Pretreatment of the incoming
untreated wastewater prior to the three trains was undertaken by coarse and fine
screening and with sand for oil and grease removal. Trains A and C were identical in
design capacity, treating 24 MLD each, while Train B had an original capacity of
12 MLD. Shortly after assuming operations at the Verziano WWTP in July 1995, the
water company ASM Brescia undertook a series of evaluations and identified problems
relating to additional discharges. The most significant problem was plant capacity,
which was about two-thirds of that required. In response, several important
projects were undertaken over the subsequent 5-year period (1995–2000) to remedy
these issues.
The principal remedial step was the upgrading of Train B, the oldest of the three
trains, from 12 MLD to 38 MLD annual average flow capacity by converting the