Activated carbon filters are very commonly used in water treatment plants to remove contaminants. They have a bed of Carbon where containments are attached by a phenomenon called adsorption.

The purpose of an activated carbon filter is to remove free chlorine, an oxidizing agent that will damage most resins and reverse osmosis membranes in a water treatment system.

Activated carbon may also remove organic material. however, the flow rates are much slower than for the removal of chlorine. Generally, the performance is not known until after the unit has been installed and operated for a while, and the bed must be replaced frequently. Other organic reduction technologies are usually indicated where positive reduction is required.

During operation, water enters the filter under pressure and is distributed by an inlet distributor over a bed of activated carbon.

The bed of activated carbon sits on the hub/lateral under the drain assembly at the bottom of the vessel. The hub/lateral assembly collects the filtered water and directs it out of the vessel through the service outlet.

If the bed becomes clogged with particulate matter or becomes compacted (as evidenced by a high-pressure drop), the filter is cleaned with a backwash cycle. Valves redirect the flow of water into the vessel through the service outlet. The water flows up through the vessel that contains the activated media bed and carries all the impurities to the backwash outlet. A freeboard is necessary to allow expansion of the media bed to aid the removal of impurities. A rinse is needed after the backwash to remove the remaining particulate matter.

The following modes of operation for activated carbon filters are described below:

During operation, water enters the filter under pressure and is distributed by an inlet distributor over a bed of activated carbon.

The bed of activated carbon sits on the hub/lateral under the drain assembly at the bottom of the vessel. The hub/lateral assembly collects the filtered water and directs it out of the vessel through the service outlet.

If the bed becomes clogged with particulate matter or becomes compacted (as evidenced by a high-pressure drop), the filter is cleaned with a backwash cycle. Valves redirect the flow of water into the vessel through the service outlet. The water flows up through the media bed and carries the impurities out of the vessel through the inlet distributor and the backwash outlet. A freeboard is necessary to allow expansion of the media bed to aid the removal of impurities. A rinse is needed after the backwash to remove the remaining particulate matter.

 General Philosophy for Activated Carbon Filter

Two 100% activated carbon filters shall be supplied to treat the desalinated water. Activated carbon filters shall be provided with an in-line flow meter. The flow meter shall be positioned upstream of the activated carbon filter.

Both filters are generally operated on normal flow. When activated carbon needs to be replaced in one filter, the throughput of the unit will be momentarily reduced. This flow reduction does not affect the end user since the storage drum is provided as a surge drum in between.

The activated carbon filter shall be about 1~2m (1m minimum). A maximum of 5 volumes of water shall be treated per hour volume of carbon bed.

The activated carbon bed shall be backwashed once a day using the water from the Demin plant supply pumps. The backwash space velocity shall be 12.73/hr and LV is 20m/hr. And achieve a 20~25% bed expansion (by volume). The duration of the backwash shall be 10min.

The activated carbon filter shall be operated automatically in both normal operation and backwash mode. The backwash sequence shall be started on a time basis by the control system.

However, if the differential pressure transmitter is reached at a high setting point, the backwash process shall be launched manually by the operator.

The backwash water also uses the water from the Demin plant supply pump at counter-current flow in a way to achieve a 20~25% bed expansion by volume. The backwash liquors shall be directed to the non-contaminated (NW) water sump. When the backwash sequence is in operation, flow in the forward direction shall be stopped.

When the backwash sequence is in operation, the flow in the forward direction shall be stopped.

The next process, the Mixed bed exchanger package, shall be shut down.

Especially, Backwash of the filter shall not be possible during mixed bed regeneration.

After one filter is finished for the backwash process (three(3) steps), the other filter starts to backwash process in due sequence.

 

Backwash

Start of regeneration of Activated Carbon filter;

• When the setting time of the Activated Carbon filter is reached; the Automatic Backwash
• When the differential pressure of the Activated Carbon filter exceeds setting values; Manual Backwash

The designed differential pressure is max 1.0barg.

Settle

The process is to stabilize the raised media during backwash.

Rinse

The water supplied from the filtered water pump is supplied to the vessel top distributor and drained to rinse the outlet valve of the vessel bottom.

Maintenance for Activated carbon filter Unit

Service Inlet Piping

1. Inlet Pipe

The service inlet piping is connected to the inlet distributor. The inlet piping and inlet isolation valve must be sized to handle the backwash flow, which is higher than the normal flow rate unless the backwash water is fed from a separate source. With a separate source, the inlet piping and inlet isolation valve are sized for the service flow rate, and a backwash inlet isolation valve is added to the unit.

Inlet Distributor

The inlet distributor has two basic functions: to provide a uniform distribution of service flow to the media bed and to collect the backwash water. The inlet distributor consists of a main header pipe with smaller pipes, or laterals, distributing water away from the header. At the end of each lateral, the flow is directed up into a splash plate.

Media

A configuration of filter media in an Activated Carbon filter is as follows (from bottom to top):

(For one(1) filter)

MEDIA	BED DEPTH	SIZE	DENSITY	VOLUME
Carbon	1,570 mm	8~30mesh	0.5	7,100 liter

• Media Lifetime (a period of change):      1 year
• Activated carbon is supplied into the top manhole of the A/C filter by the media loading system
• It puts in the order of their size: Carbon.
• Media is out of the filter from the side manhole when it is reached at replacement time

Maintenance & checkpoint

ITEM	CHECK POINT	FREQUENCE	REMARKS
A/C Filter	1. Media quantity is appropriate for service?
	2. How does the media pollute?
	3. Is pressure drop the normal condition?
	1. Is the operation working normally?
	5. Is the extraordinary particle in the filter?
Auto valve	1. Is the operation worked normally?
	2. Where is the air leaked in piping line?
	3. Is the service air pressure regularity?
	4. Is the water leaked in connection part?
Instrument	1. Is Indicator/Operation worked normally?

Trouble Symptoms & Source and Maintenance

As the operation is suddenly stopped during the operation,

Trouble Source	Maintenance
HMI trouble or shut down	To check HMI system
Control valve/Manual valve closed or opened	To check the suitable operation status for valves
Make-up water pump is not serviced	To check the main pump

As the backwash water is not supplied at backwash filter,

Trouble Source	Maintenance
Make-up water(Desalinated water) is not service or flow rate is low	To check the suitable operation status and supply backwash water of 40㎥/hr