Bioreactor Foaming

1      Types of foam

There are two basic types of foam:

1.1      Chemical foams

Chemical foams are typically

·         Light in texture - the bubble size may range from “bath like” bubbles to dense 1 – 2 mm bubbles.

·         Easy to break down with water or by leaving to stand.

·         Range in colour from pale brown to dark brown.

Chemical foaming events typically

·         Are of short duration.

·         Sudden to start and stop.

·         May be regular in occurrence (at a certain time of the day).

·         Are due to ship operations involving cleaning chemicals.

1.2      Biological foams

Biological foams are typically

·         Dense.

·         Stable when subjected to mechanical or physical abuse.

·         Range in colour from black brown though to mid brown

Biological foaming events typically

·         Are persistent – may take days to stop.

·         Occur as a result of longer term environmental problems with the bacteria.

2      Sources/locations of foam

Foaming is caused by the application of energy to the body of the biomass or to a liquid surface. Typically this energy may be in the form of:

·         air passing through the liquid body as in the main bioreactor tanks, or

·         free flow of liquid coming into contact with a free liquid surface as in the bioreactor first stage.

Thus typical areas where foam may develop are:

1.         Main bioreactor first stage – from aeration and raw water falling onto the liquid surface.

2.    Main bioreactor second stage – from aeration.

3.    Screenings tank from free falling liquid out of ISF.

4.    Filtrate tank – from free falling liquid out of ISF.

Of these, 2, foaming from the main bioreactor tanks and 3, foaming from the screenings tank are the most common but all may be experienced from time to time.

3      Preventing foam

3.1      By design

Certain design precautions will have been taken to prevent foaming occurring. These will typically be in place on any MBR installation. Standard design considerations may be:

·         Baffling free falling flows to direct the liquid energy onto a wall and away from free surfaces or pump suctions.

·         Minimising the amount of air used at any given time to reduce aeration energy in reactors.

3.2      By operation

The operator may use any or all of the following strategies to reduce the risk of foaming.

·         Using low foaming characteristic chemicals for cleaning and other applications which pass the chemical into the grey or black water system.

·         Ensuring that chemicals enter the system from cleaning applications in a controlled way and are not allowed to form high concentration sin the grey water tanks.

·         Management of the grey water so that flows with typical high foaming qualities such as the laundry waste are passed into the grey water system over as long a period of time as can be managed.

·         Maintaining the biological environment within the recommended design limits set out in the O&M manual.

·         Ensuring the loading onto the system is maintained as much as possible within the design limits set out in the O&M manual.

·         Maintaining log sheets to monitor long term changes in the operating parameters on the system.

Each of these will act either to reduce the chance of a foaming event or enable monitoring of the system to predict a change in the biological environment that may promote foaming.

4      Controlling foam

Foaming may be controlled by:

·         Reducing the energy available to create the foam.

·         Adding antifoam to the system to control the foam as it is created.

MEASURES TO BE TAKEN IN THE EVENT OF FOAMING.

Design measures will have been taken to automatically control foaming events and to ensure that any foam produced is retained within the system, however at times these may not be enough and the operator may have to intervene. In these instances the following action should be taken:

1.    Remove the source of energy creating the foam by:

·         Switching of the interstage filter and ISF feed pump[1].

·         Switching off the process blowers.

Note: Either of these actions will put the unit into fault mode and close the inlet valves The operator may wish to monitor levels in the black and grey water storage to ensure the operation of the vessels waste water system is not compromised.

2.    Contain foam if necessary by replacing access hatches on tanks – access hatches on the screenings or filtrate tanks will often be where foam will emerge.

3.    Use any pumps operating in manual to reduce the level in tanks affected by the foaming – this applies particularly to foaming occurring in the screenings or filtrate tank.

4.    Switch on the antifoam pumps if these are not already running.

5.    Wait for foaming to subside

6.    Once the unit has stabilised and the foaming has stopped run one small blower in hand and start ISF and pumps - note adding a cap full of antifoam to the screenings and filtrate tank will help to prevent further foaming.

7.    Bring the plant back up by running additional blowers in hand until the desired number of blowers is running.

8.    Once the plant is operating in a stable way put the blowers into automatic operation.

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[1] Foaming from the ISF is a clear indication of chemical foam. In this instance hotel services should be contacted to identify potential chemical management measures to reduce future risk of foaming occurring.

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