Low risk of clogging, since water and air (or water and N2 produced in denitrification) are flowing in the same upward direction.
	Good O2 transfer within the filter bed.
	Highest O2 concentration where it is needed most, e.g. at the raw water inlet.
	Good removal of N2 produced during denitrification.
	The filter is filled with media, which serves as support for a biofilm. Usually, the biofilm is quite thin (only a few hundred £gm), and highly active.

Since larger size particles are difficult to remove during backwash (although a specific backwash sequence is used for this), fine screening (2-3mm) is recommended before the raw water enters the biofilter to avoid problems.

Two ways are used for air injection into the filter: under the nozzle floor or above. Injection under the nozzle floor has the advantage of lower investment cost, since no additional nozzles for air injection are required. In addition, air injection nozzles above the nozzle floor, embedded in the filter media are difficult to replace in case if failure. So air injection without such nozzles represents less risk of equipment failure.

On the other side, additional nozzles or diffusers above the filter floor tend to give slightly higher O2 transfer rates, thus reducing operation cost. BHU proposes to test both methods on O2 injection under the specific site conditions of Hong Kong.

In a general sense, O2 transfer efficiency under wastewater conditions in BAF filters is in the order of 15 to 22%, depending to air/water ratio, O2 consumption within the filter, and filter velocities of air and water.

Backwash is done in regular time intervals to remove sludge and biofilm produced within the filter media.

Depending on the raw water quality (concentration of BOD, SS, etc.) and the operation parameters of the filter (filter velocity, grain size of media), backwash is done every 16 to 36 hours. In most cases 24h or 36 hours is the usual time interval.

Usually, backwash is initiated either by the elapsed time since the last backwash. However, to protect the filter from clogging, the pressure underneath the nozzle floor is used as control parameter. If this pressure becomes higher than a pre-selected value, a backwash is initiated.

Usually, the filters are designed and operated in a way not to exceed 2.5 to 3.0 of total pressure loss (including piping, nozzle floor etc.).

The backwash is composed of several, well studied sequences of lowering of the water level, air backwash only, air + water backwash and water only backwash.

Backwash velocities for water are between 20 and 35m/h, depending on the filter characteristics.

Backwash water consumption and wastewater production is in the order of 10 to 11m3 per m2 of filter and per backwash.

Air backwash velocities are 80-100Nm/h.

The length of each backwash, including valve operation, is in the order of 35 to 45 minutes.

Backwash water typically contains 500 to 1000mg/l suspended solids. These solids are rather easy to settle out, since the biofilm process produces compact flocs, with sludge volume indices in the order of 50 to 80ml/g.

Although it is not very easy to give treatment performances of BAF plants in a generalized way, the following effluent quality can be expected for normal municipal wastewater, if BAF technology is used either downstream of a pre-treatment, or as multi-stage biofilters.

(*)6mg/l with post-denitrification using methanol

Control of BAF is always done by a centralized control system, in order to minimize manual operation and process risk. This is especially true for filter backwash.

	Different flow rates to the plant are treated by
	Taking filters into standby below a minimum flow velocity Taking filters into operation above a defined maximum flow velocity or pollution load per filter distributing the raw water evenly to all filters in operation