By Jenn Watt
Manatee County, Florida is home to 43 parks, two golf courses and miles of beautiful beaches. The Lake Manatee Water Treatment Plant processes 317,975 m3/day of drinking water and serves 350,000 residents. It uses the multiple barrier approach to ensure the safety of the water. This includes source protection, optimized particle removal at the purification plant, and appropriate disinfection.
Drinking water is a blend of purified groundwater and purified surface water. In 2017, an average of 61,096 m3/d of deep groundwater and 92,591 m3/d of surface water was used. The groundwater is drawn from seven 366-m deep wells. This water is then pumped through a 91.4-cm pipe approximately 21 kilometres to the water treatment plant.
Surface water is taken from a reservoir. Water supplies are protected through purchasing land in the watershed area. There are about 35,000 acres that are publicly owned. Additionally, the quality of the drinking water is protected through maintaining the filtration technologies employed by the plant’s three treatment trains. They operate two for treating surface water at a capacity of 204,412 m3/day, and one for treating groundwater at a capacity of 113,562 m3/d.
The multimedia filters in the surface water trains were approaching the end of their useful life so treatment options were evaluated and it was decided to upgrade to an immersed ultrafiltration (UF) membrane system. After issuing a bid, SUEZ–Water Technologies & Solutions was chosen as the membrane system supplier to upgrade their treatment system.
Benefits of submerged hollow-fibre membranes
With a compact design and high-filtration capability, hollow-fibre, ultrafiltration membranes offer several significant benefits over conventional water treatment technologies, including: lower energy costs; lower life cycle and maintenance costs; smaller footprint and related construction costs; and, simplistic system design.
As concerns over energy efficiency increase, ultrafiltration technology becomes more appealing, since the systems can process more water using less energy than other traditional systems.
A comprehensive UF system design
Taking into consideration the anticipated raw water quality, SUEZ developed a state-of-the-art immersed UF membrane system for the surface water trains at the water treatment plant. The solution includes 12 membrane trains of ZeeWeed* 1000 UF membranes, with a two-stack design, to be retrofitted into existing sand filter basins. The ZeeWeed 1000 hollow-fibre design increases filtration surface area and therefore capacity, while remaining in the same plant footprint. ZeeWeed membrane construction centres around the microscopic pores on the polymer membrane surface which form a physical barrier to contaminants, preventing passage through to the treated water.
The small pore size of the UF strands, typically 0.02 microns, ensures that particulate matter, including Cryptosporidium cysts, Giardia cysts, bacteria, suspended solids or other contaminants of concern, will not pass into the treated water stream.
To clean the membranes, a regular backwash of the system dislodges the collected contaminants from the membrane surface and routine flushing discharges the concentrated water.
Thousands of individual hollow fibres comprise one ZeeWeed 1000 module. The ZeeWeed modules comprise cassettes, and the cassettes create trains. With 12 trains, the Lake Manatee Water Treatment Plant will be one of the world’s largest retrofits of a sand filter to a membrane filtration system, capable of processing 204,412 m3/day.
The system is expected to meet all of Manatee County’s design criteria. To save on permeate pumping costs, SUEZ is also incorporating a partial siphon operating mode. When the transmembrane pressure is less than approximately 6.5 pounds per square inch (psi), the permeate pumps, which pull the water through the membranes, will be bypassed and the UF system will operate in siphon mode.
In this mode, gravity acts as the driving force, due to the elevation difference between the membrane basins and the blend chamber. The blend chamber is where permeate will be blended with filtered water from the groundwater train. The blended stream will be treated for disinfection and stabilization and then sent to storage in clearwells before being distributed to the residents.
In a separate project, the plant implemented biological roughing filter (BRF) treatment ahead of the surface water treatment trains to replace existing powered activated carbon (PAC) for taste and odour control. To provide additional flexibility, the existing PAC system was retained to be used in tandem with the BRF system, if needed.
The water fed into SUEZ’s UF membranes will first be subjected to existing coagulation, flocculation, and sedimentation, then screened to protect the membrane. Sodium hypochlorite and sodium hydroxide will be added for pH adjustment. Quality goals for the membrane feed water quality are: true colour < 10 platinum-cobalt units (PCU); turbidity < 2 nephelometric turbidity units (NTU); pH of 6.2.
Highly efficient performance is only possible with robust support, enabled by quality ancillary equipment. At the Lake Manatee Water Treatment Plant, this equipment includes backwash storage and pumping, clean-in-place (CIP) tanks and pumps, an ion exchange system to soften water for the CIP system, air compressors, and an electrical room.
The full SUEZ–Water Technologies & Solutions membrane system is designed for 96.3% recovery at design capacity, with backwash water recycled within the plant and spent chemical solution disposed to the sewer after neutralization.
Jennifer Watt, P.Eng, is with SUEZ Water Technologies & Solutions. This article appears in ES&E Magazine’s August 2018 issue.