The foundation of peak performance

Humble underdrain is key to filtration efficiency – and longevity - in New Britain, Connecticut. By Jay Richardson, New Britain Water Company, Chief Operator

New Britain, Connecticut, sometimes called “Hardware City” because of its history as a manufacturing center and as the headquarters of Stanley Black & Decker, is a small city about nine miles from Hartford. Infrastructure followed industry in this working-class area, including municipal water service. Origins of the current New Britain Water Company date back to 1856 when Frederick T. Stanley (the City’s first mayor & founder of the Stanley Works, now known as Stanley-Black & Decker) had a study performed about the feasibility of securing a water supply to the city. Shuttle Meadow Reservoir was created and became the primary water source. It has been dammed a number of times over the years to reach its capacity of 1.3 billion gallons today, and along with numerous other smaller reservoirs and bodies of water, feed into the water treatment facility.

New Britain’s original water treatment plant was built in 1940 and hit its peak service levels in the mid-1960s, when manufacturing and population were at an all-time high. While it served the city’s needs for more than 60 years, the time came for New Britain to bring their drinking water treatment plant up to date. A new $60 million water treatment plant was completed in 2004 to accommodate up to 22 MGD, although since then, daily capacity averages about 10 MGD for 18,000 service points (85,000 people). 

A conventional treatment plant, the New Britain Water Company (NBWC) uses ozone as a pre-oxidant, followed by coagulation (with polyaluminum chloride), flocculation, filtration, and chlorination for final disinfection. The main objective of pre-oxidation with ozone is to remove the organic precursors of disinfection byproducts that can form when chlorine is used for disinfection.

Ozone pre-oxidation also provides numerous other benefits:

  • Parasite (cryptosporidium and giardia) control and preliminary disinfection of the raw water
  • Color, taste, and odor removal
  • Iron and manganese oxidation to facilitate their removal in downstream solids removal processes
  • Flocculation and clarification enhancement with reduced coagulant demand
  • Algae growth control
  • Improved removal of organic contaminants (including precursors of disinfection byproducts) via direct oxidation and by converting recalcitrant organic matter into soluble, highly biodegradable material that is removed in the downstream biologically active DE NORA filter.

Removal of solids occurs in three high-rate inclined plate settling tanks, after which, the water flows through five filter cells, each 672 sq. ft., which comprise the DE NORA DE NORA® U-Block™ underdrain, a biologically active filter which currently processes about 2.07 gallons per minute per square foot. Filter media includes six inches of silica sand with 42 inches of activated carbon on top. Once through the filter, hydrated lime for pH and alkalinity, sodium hypochlorite and fluoride are added and the water moves via three finished water pumps to a 4.25 MG clear well. The whole process, from ozone through DE NORA filter, was specified with the goal of removing as much organic matter from the water, as early in the process, as possible. This reduces the need for large amounts of chemicals to be applied to the water, which in turn keeps levels of disinfection byproducts low and meets federal regulations. Since the addition of ozone to the treatment process, the levels of regulated disinfection byproducts have been well below the maximum allowable contaminant levels: < 60 ppb for Haloacetic Acids (HAA) and < 80 ppb Total Trihalomethanes  (TTHM).

From the clear well holding tank, the clean water is gravity fed to the city and pumped to high service areas. New Britain is expected to continue to grow and the facility has the opportunity to utilize a higher rate of water treatment in the future.

The DE NORA DE NORA® U-Block™ underdrain has performed flawlessly throughout its 16 years in operation. From the moment it was commissioned, until today, it has operated reliably with no maintenance issues or repairs needed. The DE NORA DE NORA filter, as compared to the old plate-steel filters which used various grades of sand and gravel and lasted 65 years, felt like brand-new technology to NBWC. Turbidity in the water prior to reaching the DE NORA DE NORA filter is normally 1.0 to 1.5 NTU, and after the filters remove 99.9% of the particles, the turbidity drops to an excellent 0.02 – 0.03 range NTU.

Efficient media cleaning has been instrumental in keeping the DE NORA filters operating at peak performance, especially considering that the filters are operated in a biologically active mode. Routine backwashing of the gravity filters removes solids collected in the media void spaces and controls biofilm inventory on the surface of the media. The DE NORA DE NORA® U-Block™ provides superior distribution of backwash air and water. Air is distributed evenly across the entire filter bottom area to scour the media and to provide an air lift, which, in combination with water, releases solids from the media. A high-rate water-only backwash step is then applied to fluidize the media and remove solids from the filter.

Other features of the DE NORA DE NORA® U-Block™ that made it attractive for this project were:

  • Lightweight and easy-to-handle; reduced installation costs
  • Snap fit, single gasket, bell and spigot joint assembly ensures a tight fit with no leaking
  • Reduced installation time and costs due to simple assembly
  • Long maintenance-free life
  • No moving or wearing parts
  • No materials used that are subject to corrosion
  • Easy-to-adapt to potential modifications in backwash scheme
  • Even air and water distribution across the entire filter bed to eliminate development of dead spaces and mudball formation in media
  • Low head loss and non-clogging underdrain – reduced risk of failure

One feature that is especially effective and unique is the usage of S Plate media retention caps. This technology replaces conventional graded gravel layers and prevents media loss through the filter underdrain, while also increasing the available filter headroom. The S Plate is manufactured of precisely sized and carefully sintered HDPE beads that ensure even distribution of backwash air and water for increased run times and lower operating costs. Some of the reasons DE NORA underdrains and S Plate technology in particular were specified for NBWC were the fact that S Plate media retention plates were easy to install and removable, they reduce the depth requirement for media, which is particularly beneficial for shallow filters, they contribute to uniform backwash flow distribution at a headloss similar to 300 mm (12”) gravel depth, and they have an excellent performance record in similar applications since 1996 due in large part to the strict quality control measures taken during fabrication and installation. The S Plates retain media down to 450 micron (0.45 mm) particle size, do not leach chemicals into the drinking water supply and also simplify media replacement – something NBWTP just completed for the second time in the history of the operation of the plant, with no issues or complications. A full inspection of the DE NORA underdrain system components revealed that it continues to operate as expected after 16 years of operation and appears to be in perfect condition.

A quiet city in Connecticut with a seamless water treatment filter underdrain operation isn’t going to grab a lot of headlines, but isn’t that what most water treatment plant operators wish for on a daily basis? Safety, reliability, excellent design and many years of solid performance – that’s something that Hardware City does not need to fix!