Page 273 - Membranes for Industrial Wastewater Recovery and Re-Use

242 Membranes for Industrial Wastewater Recovery and Re-use

Table 5.6 Cost baIance for different brine treatment options (based on 1990 data)
cost 2 50 gpm 90 gpm Crystalliser EDR/RO
evaporator evaporator system
Capital and 3 600 000 1950000 1 370 000 750000
installation (S)
Energy ($ dap') 1471 535 148 300
Maintenance ($ day-') 100 54 38

production rate of 5000 tons per year equates to a water usage of 500 000 m3
yearp1. The production process results in a high strength wastewater (COD =
4390 mg 1-l) at a temperature of 75-85°C and a high pH. Traditionally the
effluent from this process has been discharged into the sewer and the water
supplied from a local spring requiring 1.5 x 106 m3 of gas to heat it to the
required temperature annually.
European legislation is increasing regularly such that paper mills are being
forced to minimise their energy and water consumption. The driver for reuse in
this case was the combination of compliance with current and near-future
legislation together with the financial benefits of reduced water intake, discharge
costs and gas supply. One of the most important issues in this specific case was
the reuse of both the water and heat from the process.

5.4.2 Description of system
Flow from the production facility initially enters a storage tank prior to passing on
to adissolved air flotation plant at a rate of 10 m3 h-l (Fig. 5.9). Bubble production
is achieved by releasing a super saturated solution of water and carbon dioxide-
rich air. This has the added benefit of controlling the pH such that at a ratio of 7
m3 of gas to every 1 m3 of wastewater the pH decreases from 11 to 8.2.
The flow then enters a side-stream membrane bioreactor with a working
volume of 200 m3 and a hydraulic retention time of 22 hours. The bioreactor is
operated at a MLSS of approximately 20000 mg l-l, equating to a sludge
production of less than 0.02 kgSS d-l. The bioreactor is configured with a side
stream loop of 8 mm tubular PVDF membranes rated at a pore size of 0.04 pm.
The MBR is designed to treat an average flow of 9 m3 h-l and contains 82.5 m2 of
membrane configured in 3 trains of 6 modules. The membranes are operated at a
mean TMP of 3.5 bar delivering a mean flux of 120 LMH at a cross-flow velocity
of 3.5 m s-l. The membranes have not required cleaning in the first year of
operation but when necessary they will be cleaned by a mixture of anionic and
cationic surfactants. The membranes have an expected life of 3 years but with
the minimal cleaning required so far it is hope this may be significantly extended.
The plant includes an internal heat exchange loop whereby hot incoming flow
is cooled to the required temperature for bio treatment whilst at the same time
increasing the temperature of the product water stream. This has a direct benefit
of reducing the heat required to produce water at the required temperature for
processing the raw cotton.