Filtration Manufactured Treatment Devices

Plan view of an upflow filtration MTD with filter cartridges. This example for the U.K provides four separate stages of treatment for influent water including: 1) oil retention, 2) chemical separation, 3) filtration and 4) sedimentation (Source: 3P Technik UK Ltd, 2023).

Overview

Filtration Manufactured Treatment Devices (MTDs) are structures with one or more chambers containing filtration media, membranes and/or filtration cartridges that remove solids and debris/trash from runoff. Some units may have oil separation functions and pretreatment chambers for coarse sediment and debris. The filter components are designed to remove the coarse and fine sediment fraction of suspended solids and associated pollutants in stormwater runoff and may also target removal of dissolved pollutants such as phosphorus or metals through biological and/or chemical processes.

Example of a high flow curb inlet filtration MTD with proprietary soil media installed and monitored at the Kortright Centre in Vaughan, Ontario. (Source: STEP, 2020).

Application

Filtration MTDs are suitable for pre-treatment or may be installed as stand alone if third party performance testing shows that they achieve enhanced level water quality protection (i.e. lower 95th percentile removal efficiencies of 80% or greater).

Since Filtration MTDs contain filters that restrict flow rates, they are normally designed for much lower hydraulic loading rates than Oil-Grit Separator MTDs and have longer detention times. Flow restriction occurs due to the small pore size openings of filters and the clogging of pores and/or the formation of films either on the surface of the filter or within the filter matrix filter. For this reason, inspection and maintenance intervals for Filtration MTDs are normally shorter than for OGS MTDs. The method of maintenance typically involves sediment removal with a hydro vacuum truck as well as filter cleaning or replacement.

Further guidance on how the data and information generated from verified laboratory testing of OGS technologies should be interpreted and factored into regulatory agency approvals and procurement decisions for stormwater management is provided in this publicly available specification Guidance on the Use and Application of Results from Verified Laboratory and Field Testing for Stormwater Manufactured Treatment Devices, developed by TRCA and the Standards Council of Canada that addresses both OGS and Filtration MTDs.

Testing and Verification

Testing of Filtration MTD performance must be based primarily on field monitoring. Field testing is particularly important for Filtration MTDs because stormwater sediments consist of a mixture of cohesive silts, clay, sand, and may include organic matter, emulsified oils, fine debris, bacteria and other pollutants that build up on filter surfaces and/or penetrate into the filter structure(s) resulting in clogging dynamics that may vary significantly across different filtration system designs. These clogging processes are not easily replicated in laboratory testing due to limitations on the type of test sediment that is used (non-cohesive ground silica) and other factors.

Field testing also allows for a more accurate assessment of filter performance for stormwater pollutants other than suspended solids. These pollutants are subject to complex physical, chemical, and biological interactions that are not easy to replicate in a laboratory.

In Canada, the Canadian Procedure for Field Testing of Filtration Manufactured Treatment Devices, developed by TRCA and the Standards Council of Canada stipulates field testing requirements for Filtration MTDs. This document was developed based on the Technology Assessment Protocol – Ecology (TAPE). TAPE provides a robust and scientifically defensible field evaluation methodology for proprietary and non-proprietary stormwater treatment technologies that is well regarded by stormwater practitioners, having been developed and continuously updated over the past two decades through an extensive third-party expert peer review process.

It should be noted that several jurisdictions in Canada do not accept filtration MTDs without field monitoring due to the aforementioned concerns, even if a lab test has been verified through the ISO 14034 ETV process.

Performance Results

Existing ‘General Use Level Designation’ certification of Filtration MTDs through the Washington TAPE process may satisfy ETV testing requirements in Canada provided that the design of the tested unit has not changed, TAPE requirements were followed, and hydraulic laboratory testing is used only to augment, rather than replace field testing requirements. In some cases, additional testing and data analysis may be required to support the ISO 14034 verification.

A list of TAPE approved Filtration MTDs with General Use Designation can be found on the Washington TAPE website, along with technical reports for each device. You can find the list of over 106+ approved devices here Emerging stormwater treatment technologies (TAPE). The following table on the TAPE website lists the devices that have received a use designation through the TAPE process. These use designations can be divided into three separate Use Level Designation (ULD) categories:

1. General Use Level Designation (GULD): Known technologies whose given evaluation report showcases that they can confidently achieve Washington state Technology Assessment Protocol - Ecology's required performance goals.
2. Conditional Use Level Designation (CULD): These are technologies that should most likely and feasibly be able to meet stated performance goals, but the developer most conduct additional verification field testing (within a given time period) to achieve General Use Level designation.
3. Pilot Use Level Designation (PULD): Newly piloted and promising emerging technologies whose use is specific to a given project's field testing requirements. Requires further verification testing though.

Scaling

Appropriate scaling ensures that performance results from the model tested and verified through TAPE and/or ISO 14034 ETV can be confidently applied to other model sizes for the same MTD. Buyers should ensure to ask vendors to provide sizing data and calculations to confirm that the proposed unit is scaled appropriately. Verifications conducted in 2023 or later will include information on unit scaling in the publicly available verification statement.

The scaling rules, as stated in the Canadian Procedure for Field Testing of Filtration MTDs, are as follows:

"Scaling the tested filtration MTD to determine other model sizes and performance without completing additional testing is acceptable provided that:

1. The depth, composition, bulk density and gradation of media remain constant. Nominal pore size of membrane based filters remain constant;
2. The ratio of the system treatment flow rate to effective filtration treatment area (filter surface area) is the same or less than the tested filtration MTD;
3. The ratio of effective sedimentation treatment area to effective filtration treatment area is the same or greater than the tested filtration MTD; and
4. The ratio of wet volume to effective filtration treatment area is the same or greater than the tested filtration MTD
   

Sizing

Appropriate sizing of Filtration MTDs is more complex than for Oil-Grit Separators because of the wide variation in different filtration device designs. Assessing whether performance of Filtration MTDs meets performance objectives for the site based on verified testing reports will require the following information:

• Historical rainfall records (≤ 1 hour recording intervals, 15 minute preferred where available) from the nearest long term meteorologic station for months when temperatures are predominantly above 0°C. Data should be filtered to account for the initial abstraction factor, which represents an assumed loss occurring during the initial phases of a storm and ends when the total rainfall depth equals the abstraction factor (e.g. 2 mm);
• Site drainage area size, impervious cover percentage and site runoff coefficient calculated based on pervious and impervious cover runoff coefficients relevant to the site drainage area (e.g. slope, soil texture) and jurisdiction;
• Treatment objective (normally denominated as a removal efficiency for water quality parameters of concern, but some jurisdictions may use effluent concentrations either in addition or instead of removal efficiencies) and location in drainage system (i.e. stand alone or as pretreatment to other downstream BMPs); indicate whether upstream flow control is provided, and if so, the type (e.g. parking lot storage, roof storage, pipe storage);
• Proposed model sizes and specifications (diameter or dimensions, internal sedimentation area, manufacturers recommended maintenance sediment storage depth, chamber depth, filter type and number, media composition – if relevant);
• Verification test results (from field monitoring through TAPE or equivalent): lower 95th percent confidence interval of mean pollutant removal efficiency for each water quality parameter; upper 95th percent confidence interval of mean effluent concentration for each parameter; lowest measured bypass flow rate, head loss data, and other relevant operational parameters;
• Design TSS runoff concentration for the site or jurisdiction

As with OGS, the Rational Method can be employed to estimate flow rates discharged to the MTD for the site in question based on historical rainfall records and the selected recording interval (≤1 hour, 15 minute preferred where available). These flow rates are in turn used to calculate hydraulic and sediment mass loading to the selected Filtration MTD model (TRCA, 2023). In general, sizing should ensure that at least 90% of the annual runoff volume during an average year is treated (i.e. does not exceed the system treatment flow rate as defined in Table 2 below).

The technique of removing pollutants from runoff as it infiltrates through the soil.

Deposition of material of varying size, both mineral and organic away from its site of origin by the action of water, wind, gravity or ice.

Settling-out or deposition of particulate matter suspended in runoff.

The technique of removing pollutants from runoff as it infiltrates through the soil.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

Initial capturing and removal of unwanted contaminants, such as debris, sediment, leaves and pollutants, from stormwater before reaching a best management practice; Examples include, settling forebays, vegetated filter strips and gravel diaphragms.

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

Sustainable Technologies Evaluation Program

The total mass of a pollutant entering a waterbody over a defined time period.

The net amount of something (e.g. chemical, such as phosphorus), calculated as the product of concentration and volume in a given time. Some BMPs significantly reduce loading of pollutants to the environment by reducing volume more so than concentration.

The temporary storage of stormwater to control discharge rates, and allow for sedimentation.

Systems designed to remove trash, debris and some amount of sediment, oil and grease from stormwater runoff based on the principles of sedimentation for the grit and phase separation for the oil.

Toronto and Region Conservation Authority

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

1. A mineral soil separate consisting of particles less than 0.002 millimeter in equivalent diameter. 2. A soil texture class. 3. (Engineering) A fine-grained soil (more than 50 percent passing the No. 200 Sieve) that has a high plasticity index in relation to the liquid limit. (Unified Soil Classification System).

The General Use Level Designation (GULD) is provided by the Washington Department of Technology to all technologies that meet the performance goals set out in the TAPE protocol. Performance requirements include 80% TSS removal for influent concentrations from 100-200 mg/L and effluent concentrations <20 mg/L for influent concentrations from 20 to 100 mg/L. See Table 2 in the TAPE protocol for more information.

The General Use Level Designation (GULD) is provided by the Washington Department of Technology to all technologies that meet the performance goals set out in the TAPE protocol. Performance requirements include 80% TSS removal for influent concentrations from 100-200 mg/L and effluent concentrations <20 mg/L for influent concentrations from 20 to 100 mg/L. See Table 2 in the TAPE protocol for more information.

The General Use Level Designation (GULD) is provided by the Washington Department of Technology to all technologies that meet the performance goals set out in the TAPE protocol. Performance requirements include 80% TSS removal for influent concentrations from 100-200 mg/L and effluent concentrations <20 mg/L for influent concentrations from 20 to 100 mg/L. See Table 2 in the TAPE protocol for more information.

A hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.

The total surface area upstream of a point on a stream that drains toward that point. Not to be confused with watershed. The drainage area may include one or more watersheds.

A system flow of gully inlets, pipes, overland flow paths, open channels, culverts and detention basins used to convey runoff to its receiving waters. City of Toronto 45 Wet Weather Flow Management November 2006

Total suspended solids

This formula is used to calculate peak rates of runoff when designing drainage LID BMPs in urban area. It is useful in estimating runoff on relatively small areas (roofs, parking lots, medians, road right-of-ways, etc.)

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.