Chadson Granular Filters
Mechanical & Chemical Filtration for Swimming Pool Water, Industrial Process Treatment, & Waste Water Industries



The fiberglass pressure filters specified herein shall be the medium to high rate type as manufactured by CGF or approved equal. The filter manufacturer shall be regularly engaged in the production of water filtration systems with a minimum of ten (10) years experience with the manufacture of fiberglass pressure filters.

The filter system for the ………………………………..shall comprise ………. only Model No …………… filters that provide a total filter area of …square meters. The filter system shall operate at a filter rate of……….m3/hr/m2.

Individual filter modules shall be arranged to backwash at a rate of not less than … m3/hr/m2. With a conventional “water-only” backwash, each filter shall require a reverse flow of not less than……..l/second. Each filter vessel shall consume up to …m3 of wash water in a normal four-minute backwash. The system/process design shall be sized to permit consecutive backwashing of individual filters. When specified filter vessels shall be air scoured at a rate of not less than 32 m3/hr/m2 at 0.35 bar.


Filters shall be manufactured from FRP materials in general accordance standard procedures that recognize BS-4994, DIN 18820, and AS-2634. Vessels shall be designed and individually pressure tested to a certified test pressure of ……kPa. (The standard test pressure is 350kPa)

Filter vessels shall be adequately supported with a
skirt or number of cradles that are resin bonded to the filter shell. Filters shall be installed on a flat concrete base or plinth that is provided by others. The service weight of the filter shall be …………kgs. The plan area of the support skirt/cradle shall not be less than ……….m2.

Model …………..shall feature ……….number of access hatches as detailed on relevant shop drawings. Tank fittings shall include kinetic air bleed/vacuum breaker, screened drain cock, gauge panel, DN…………flanged inlet/outlet nozzles & identification label. Other special fittings and or requirements shall be noted as an Addendum to these Specifications.

Filter vessels shall be fabricated from approved resins in accordance with a comprehensive plan that indicates the nominal thickness, the distribution of reinforcing fibers according to type, quantity, and arrangement of the surface layers, resin type and curing mode. This plan shall recognize the requirements of DIN 18820. Vessels shall be internally lined with an impervious layer to resist chemical attack & abrasion.


Filter internals shall be manufactured from uPVC & ABS of adequate size to provide hydraulic balance in both the filter and the backwash cycle. All internal pipework shall be adequately supported and fixed within the filter to suit the intended water flow and the operating pressures.

The top distributor (overdrain) shall be arranged to disperse soiled water evenly over the filter bed and to collect all particulate matter and wash water during a backwash cycle. Distribution headers shall be fitted with patented FloModuLata’s™ and or calibrated sized nozzles. Headers with simple drilled holes shall not be accepted.

The underdrain system shall comprise all necessary uPVC headers and injection moulded laterals. Laterals shall be moulded from engineering plastic (ABS) with inward tapering V shaped openings that are in general accordance with DIN19605. Drilled or slotted pipe laterals shall not be accepted. The underdrain system shall be sized to provide an open area that relates to the cross sectional area of the filter’s inlet/outlet. Where applicable the underdrain system shall be fitted with patented FloModLata’s™. The top distributor and the underdrain system shall provide a parallel flow from influent to effluent whilst maintaining a level and effective filter bed.


Face plumbing shall be fabricated on-site or alternatively, it shall be manufactured in the workshop of an approved pipe fabricator. (Strike out whichever is not applicable). Shop fabricated piping shall be subject to trial assembly at the Filter Specifications, Continued.

manufacturing plant. The system shall be dismantled for shipping purposes into convenient sub assemblies to minimize site assembly. The piping system shall be manufactured from …………………. materials in accordance with……………………….. (Insert either project specifications or a relevant Standard).

Face plumbing connecting to the filter(s) shall be sized based on having a line velocity not exceeding 2.5 m/sec. All pipework and valves shall be suitably supported in accordance with accepted Industry practice.

Unless specified elsewhere, valves shall be uPVC butterfly type, with EPDM seat, and stainless steel shaft. All valves up to 200 diameters shall be lever operated; valves over 200 NB shall be gear operated.

When specified, individual filters shall be supplied with Chadson single lever multiport valves for backwash purposes. Alternatively, filters shall be equipped with a matrix of either four or five commercially available butterfly valves. (Strike out whichever is not applicable and note that Shunt type backwashing using clean filtered water is specified as an Option).

Backwashing shall be conducted manually, according to pressure differentials (shown on the filter gauge panel) or at determined time intervals that are established to suit the application. The backwash piping system shall include a sight glass for visual inspection of waste wash water.


The installing Contractor shall check each filters lateral system (for any transport damage) before placing any media. Filter medias shall be placed into the filter vessel by the installing Contractor. Filter media shall be as follows:-

Bottom (1st) Layer:-
Media Type, Grade, & Qty (kgs) …………………………

2nd Layer:-
Media Type, Grade, & Qty (kgs) …………………………

3rd Layer (if applicable):-
Media Type, Grade, & Qty (kgs) …………………………

Sand type medias shall consist of uniformly graded silica sand, free of all foreign material, limestone and clay. Media shall be manufactured to AWWA B100-89 Standard with a specific gravity of 2.65 and a uniformity coefficient not exceeding 1.5.

Filter media shall be leveled-out to provide a flat & total bed depth of not less than …………mm.



In this option, the filter vessels shall be arranged to be individually backwashed with clean filtered water; in this arrangement, conventional reverse flow backwashing with soiled water is NOT acceptable. Shunt backwashing shall be provided in accordance with the filter manufacturer’s Instructions.


If and where specified, the filter system shall be provided with all valves, controls, fittings, actuators, positioners, and accessories to automatically control the filter’s functions. Push button/manual initiation shall also be provided and all automatic valves must be capable of manual operation.

Automatic functioning of the backwash system shall be primarily initiated by a differential pressure differential signal with a secondary seven day/twenty four hour timer. Backwash system shall protected against excessive flow or fault via a fail-safe system that includes (but is not necessarily limited to) a timer lockout and “normally closed” valve operation. The backwash controller shall as manufactured by CGF (eg.,Elbatrol) and or approved equal



Within seven days of an equipment Order the filter manufacturer shall provide a fully dimensioned outline of the specified filter. Allowance shall also be made to provide all necessary technical data (& printed Instructions) to assist the installing contractor with the supply and co-ordination of all associated services.


The entire system shall be inspected prior to shipment to verify compliance with the fabrication drawings and the project/product requirements. Material & Pressure Test Certificates shall be collated with all necessary installation data.


Notwithstanding any contract obligations, the installing Contractor shall provide the services of a qualified technician to visit the jobsite after the installation of the filter system has been completed. The technician shall assist with the initial start-up and the plant commissioning, ensuring that correct backwash flows are established. Allowance shall also made to assist with any specific training that pertains to the filter’s operation & or maintenance.