Treatment of oil-contaminated water
Membrane bioreactor technology is being used to treat wastewater from warships, with the potential for on-board systems.
Toine Bakx, Sjoerd Boom and Hans Ramaekers present early work with the Dutch Navy.
After a long journey at sea, warships from the Royal Netherlands Navy return home to the Netherlands Naval Base in Den Helder. During the trip, the ships collect wastewater which they are not allowed to discharge. This oil-contaminated water is collected from the engine room, the bilges and from fuel/seawater displacement systems.
Until recently the wastewater was discharged from ship to shore and treated in a flocculation/flotation unit. When more stringent effluent standards were introduced in a new discharge permit, a new type of wastewater treatment was needed based on currently available, state of the art techniques. A pilot test was conducted to investigate the possibility of treating the wastewater biologically.
Compared to domestic wastewater, this water is characterized by a high salt content (sea water) and contamination with phenol, mineral oil, and polycyclic aromatic hydrocarbons. There are hardly any nitrogen components present.
To treat this wastewater requires a bioreactor in which specific bacteria are able to oxidise the various complex components present. Since treatment of wastewater with a high salt content in conventional systems results in sludge wash-out, a different technique is needed to achieve sludge retention. To increase the activity, environmental factors such as temperature, pH and availability of oxygen and nutrients must be optimised.
Membrane bioreactor
And up-and-coming technique for wastewater treatment is a combination of bioreactor and membrane separation technology. The membrane bioreactor technology is characterized by compactness, robutness and high treatment efficiency.
Figure 1 shows a schematic view of a membrane bioreactor. Two important elements can be distinguished: a bioreactor and a membrane filtration unit.
The wastewater to be treated flows directly into the bioreactor. In this bioreactor, wastewater and bacteria (biomass) are intensively mixed. The oxygen which is required by the bacteria for degradation of organic compounds is transferred into the bioreactor by aeration. The mixture of the treated water and biomass is continuously recirculated over the membrane filtration unit. In the membrane filtration unit, the purified wastewater and bacteria are separated. The filtrate is drained off as effluent and the concentrate is returned to the bioreactor. Membrane bioreactor technology differs from traditional biological wastewater treatment. Specific characteristics of the membrane bioreactor technology give the system its ability to treat complex wastewaters. These characteristics are:
High biomass concentration
By using membranes for separation of biomass the treatment is not dependent on settling characteristics of the biomass.
Therefore high concentrations of biomass can be achieved: up to 30 g MLSS/l (mixed liquid suspended solids), which is 6 to 8 timer higher than concentional systems.
This means that the volume of the bioreactor, in comparison with conventional systems, can be reduced six to eight times, or the volume can be reduced until the oxygen transfer becomes limiting.
Full retention of bacteria
By using membranes, bacterial washout is prevented. Consequently the presence of specific bacteria in the bioreactor is guaranteed. Membrane filtration is based on the principle of separation by difference in particle size, facilitated through a sheet with uniform pores. For the separation of biomass from the treated water, normally microfiltration or ultrafiltration is used, with a pore size varying from 0.01 to 1um.
The use of micro- and ultra-filtration membranes guantees complete retention of suspended solids, bacteria and viruses.
High volumetric loading rate
Due to high concentrations of biomass, a high volumetric loading rate (expressed in kg COD/m3day) is achieved. Heat is produced as a result of the bacterial activity, and this can be used to operate the bioreactor at a temperature between 30-38oC, the optimum temperature for biological processes.
Low sludge (biomass waste) production
Due to a combination of a relatively low biomass loading rate (expressed in kg COD/kg MLSS day) because of the high MLSS content, and a high temperature, the sludge age is very high and excessive mineralization takes place. Therefore llittle excess sludge is produced.
High effluent quality
Complete mineralization of influent organic compounds is facilitated through retention of high molecular weight compounds by the membranes. The retention time of these compounds is therefore independent of the hydraulic retention time, allowing a more complete degradation.
Pilot test
To test the membrane bioreactor concept for treating the oil-contaminated wastewater, a pilot plant test was conducted in a 1m3 membrane filtration unit. Over a period of 12 weeks, data were collected. At the start, pre-treated water from the flocculation/flotation unit was used. Slowly the degree of pre-treatment was lowered, resulting in increasing concentrations of mineral oil, PAHs and MAH concentrations.
During the test, the degradation of the different components in the wastewater was monitored, and the most important results of the test are presented in the table.
In Figure 2, the results of the COD concentrations during the test are shown.
As can be seen, the various components were completely oxidized. Analyses of thecomponents in the bioreactor showed no accumulation.
The effluent concentrations during the test period remained constant, even with strongly fluctuating influent concentrations and a high salt content. During the pilot tests the membrane flux stabilised at values of 100l/m2 h. Based on the pilot test the membrane bioreactor is a technically, as well as technologically, feasible process for treating the Navy’s oil contaminated wastewater. The process is stable and can easily controlled.
Full-scale membrane bioreactor
Since the beginning of April a full-scale membrane bioreactor, built by Triqua, has been in operation (Figure 3). The treatment plant is sited is a small building (11,5 m long by 6 m wide) near the harbour. The net volume of the bioreactor is 22 m3. The average capacity of the installation is 1.5m3/h, with a maximum capacity of 3m3/h. The total membrane area is 32m2.
The influent is pretreated is the flocculation/flotation unit although, based on the pilot test, the capacity of the pretreatment system could be reduced to 75%. Nutrient dosage is also used to compensate the nitrogen and phosphate for the COD:N:P ratio, to enable the bacteria to grow.
Development
The membrane bioreactor technique proved to be a very feasible method for treating bilge water of a comparable complex wastewater. The system can ge used for the treatment of water from the whole shipping sector- for example, cruise ships or private shipping. The membrane bioreactor is a compact system which can be operated on shore in combination with a bilge water collecting system in the harbour, or it can be equally well operated on board.
The suspended solids-free effluent can easily be used for any purpose that does not require high quality tap water. For this, the membrane bioreactor will be very suitable for forthcoming water saving and recycling programmes.
