Piedroba Dewaters Slurry, Returns Clean Water to Holmes Beach Canals
This project was a successful application of a total cleaning system (TCS), where the particular type of solids of the sediments permitted the dredger to avoid the costly construction of a dredged material maintenance area (DMMA), or the transportation and tipping fees for a municipal landfill. The surface waters were cleaned in tandem, and contributed to a higher quality benthic environment in the canals.
Maintenance dredging more than 11,000 miles of rivers and waterways in Florida normally involves the dual tasks of dredging and placement of the sediments. But these sensitive environments merit adding a third task: cleaning the water itself.
The City of Holmes Beach in Manatee County, Florida, had plans and permits developed by EBA Environmental of Bradenton, Florida to remove 6,890 cubic yards of sediments from eight of their 60-foot wide residential navigable canals. The contract was won by Piedroba Marine Construction (PMC) of Coral Gables, Florida. PMC commenced dredging by mid-May 2009, and self-included a third task: cleaning the returned water extracted from the dredged sediments.
Most of the material consisted of three percent gravel, 91 percent sand and only six percent silt and clay-sized particles. PMC ran its own sieve analysis/hydrometer tests on samples, and found that the material drained extremely well if aerated. Therefore, a solids recovery system in lieu of land-fill disposal could provide an economic benefit to the city. PMC proposed that the city obtain permits from the Department of Environmental Protection (DEP) to permit the use of the clean sand recovered for re-nourishment of Holmes Beach and Ana María Island.
The Holmes Beach canals feature protruding docks, docked boats and piling, which favored the use of PMC’s 15-foot-wide LWT 300SL dredge.
The dredge is capable of pumping a rate of 3000 gpm, but could be throttled back to as low as 1,200 gpm. (A lower rate could precipitate the solids in the slurry). The dredged slurry was directed to shore via a six-inch-diameter HDPE pipeline (versus an eight inch) to maintain a higher velocity, floated on 50-gallon sealed plastic containers.
SLURRY PROCESSING EQUIPMENT SETUP
The original PMC plan was to use a minimum of three modified trucks successively to decant the dredged slurry, then return the effluent water back to the canal, and to transport the solids to a designated disposal site. This method was used in previous years by other dredging contractors. The method proved only marginally economical, even with larger volumes, and rekindled past concerns about damages to landscaping, fences, runaway pets, spills on lawns and city streets, maintenance of traffic (MOT), turbidity in the canal, and multiple silt fencing and monitoring.
An alternative method was to de-water the slurry in modified construction debris dumpsters placed at the end of each canal. The modified dumpsters were cheaper than the modified trucks. Common dumpsters hold approximately 20 cubic yards of solids. The project would require five dumpsters and 500 truck-trips to the designated disposal site 15 miles away at Bradenton. This also meant that the clean recovered sand would be disposed in a land fill. Also, the dredge could easily clog at the low rates of 800 to 1,000 gpm required to operate with dumpsters. A better solution was the use of a hydro-cyclonic separator.
Piedroba rented a “total clean system” (TCS) from DEL Tank & Filtration Systems, a division of Del Corporation of Scott, Louisiana. It consists of a portable three-stage cleaning process that cyclones and sieves the influent dredged slurry.
The dredge pumps the slurry to the TCS, which is driven by a diesel-powered electric generator that supplies 480 VAC and 250 amperes. In the first stage, the solids are cyclones and sieved to a #4 (4.75 mm) size to remove gravel, bottles, cans, large shells and other debris. The second stage removes solids down to fine sand size - #140 sieve (.106 mm). The third stage sieves to #300 seive (.053mm) and the fourth stage comprises dumpster settling tanks in series to reduce turbidity using polymers, down to NTU equal or less than 29 above baseline measurements.
FINER SIEVES UN-ECONOMICAL
Although the third stage has been used with #400 sieves (37 microns), PMC experienced significant screen clogging and equipment downtime, thus proving the finer sieves to be uneconomical.
Most of Florida’s waterways have a natural turbidity generated by plants, which secrete tannic acid as a metabolite, and stain the water with a brown-yellowish hue that darkens when exposed to air. The tannic acid is not toxic to humans, and is especially common to Florida swamps, wetlands, canals and estuaries, where they are referred to as “black-water rivers”.
Several polymers are known to be effective with tannic acid and other water-soluble contaminants. Dosage is determined via field testing (the jar test). Figure 6 shows a jar test of the suspended contaminants found in one of the City canals, where a small amount of bio-degradable polymer quickly precipitated the solids, reducing the water turbidity to NTU = 6 within a few minutes.
Common inorganic polymers include aluminum sulfate, ferric chloride and sulfate and aluminum chloro-hydrate, dosed at about 5 mg/L as a starting point. An effective organic polymer is chitosan (ground-up crustacean shells); others include sodium alginate, common starches (potatoes) and the anionic hydrolyzed poly-acryl amide; the latter use much smaller initial dosages of 0.005 mg/L.
SEDIMENT SIZE CRITICAL FOR THIS METHOD
This project was a successful application of a total cleaning system (TCS), where the particular type of solids of the sediments permitted the dredger to avoid the costly construction of a dredged material maintenance area (DMMA), or the alternative transportation and tipping fees for a municipal land-fill disposal. The surface waters were cleaned in tandem, and contributed to a higher quality benthic environment.
SOLIDS REMOVAL SYSTEM VERSUS DMMA
The TCS system in particular is an excellent remover of gravel, sand and medium silt from the dredged slurry. However, it is only economically feasible when the remaining solids (below 0.053 mm) easily and quickly settle in relatively small tanks. If the suspended solids require high use of polymers, or long settlement times, then the method may be more expensive than a DMMA.
Also important is, how knowledgeable of dredging are the regulating agency’s field personnel? These have been known to demand testing at the mouth of the discharge pipe! Instead, the owner’s consultants and designers should clearly specify where testing for turbidity must take place. The inner turbidity curtain’s pen should be sufficiently large in volume to permit dilution and the settlement of the remaining fines. The final dredging phase will clean out this small controlled area before the turbidity curtains are removed.
ABOUT THE AUTHORS: Luis A. Prieto-Portar, PhD, PE is Professor of Civil and Environmental Engineeringat Florida International University, Miami. His son, Luis A. Prieto y Muñoz, MA is president of Piedroba Marine Construction, of Coral Gables, Florida. They welcome comments and readers’ experiences with the methods described in this article, and can be reached through Jelle Prins at email@example.com or phone: 305-409-7011.