Organic Water Treatment for Sand and Gravel Operations

Introduction

It should come as no surprise that, with today's environmental regulations, Sand and Gravel mining operations experience problems associated with storm water contamination. Virtually all operations have outdoor staging and processing areas that inadvertently create small particulate material that commingles with stormwater during rain events. What is surprising is that most engineering solutions to this problem are based on 30-year-old wastewater treatment technology. These technologies make few allowances for the unique properties of storm water associated with sand and gravel operations and result in higher costs, reduced reliability, and overly complex treatment systems. Emerging and innovative stormwater treatment technologies can reduce the financial burden of environmental compliance and allow the sand and gravel industry to focus on its core business. This article will describe one such innovative storm water treatment technology (Chitosan-Enhanced Sand Filtration) and review its performance in full-scale operation at a large sand and gravel operation draining to a salmon-bearing stream.

Chitosan (pronounced ky-toe-san)

Chitosan is a biodegradable, nontoxic extract from shellfish shells used in a variety of water purification applications. Chitosan is derived from chitin (pronounced ky-tin), nature's second most abundant biopolymer and primary constituent of shellfish shells, insect exoskeletons, and fungi cell walls. Chitosan is so safe it is used by commercial aquariums to clarify the water in the aquarium exhibits and it is also used to clarify public pools and spas. In addition to its safety record, the secret of its success is the way in which it interacts with sediment particles. Chitosan creates a fibrous web linking sediment particles together in a three-dimensional matrix. When this matrix enters a sand filter it is caught in the sand but allows the water through – other polymers and coagulants form a gelatinous floc that rapidly clogs filters. Chitosan allows extremely long filtration cycles at sediment loading rates that are well above industry standards.

Facility Description

Lakeside Industries operates a sand and gravel pit and a hot-mix asphalt plant in Issaquah, Washington. The site consists of approximately 216 acres. Industrial activities that occur at the facility are aggregate mining, aggregate crushing and processing, stockpiling, asphalt recycling and hot mix plant, concrete plants, and routine equipment repair and maintenance.

Impervious surface areas amount to less than 15 percent of the total disturbed surface of the property. Impervious surface areas include the paved roads, entrance facilities, paved and concrete work areas, and several support buildings. The remainder of the site is either vegetated or exposed well-drained sand and gravel soils.

Historically the facility's surface water and process water have been diverted to a settling basin where it was periodically pumped to an infiltration pond. This situation presented two unfavorable outcomes; first was the cost of pumping millions of gallons of water over ¼-mile through piping and second, this water was needed in its natural drainage (a salmon stream) especially in the summer months when stream flows are low. The solution was to install a chitosan-enhanced sand filtration system with discharge to the natural drainage course. The treatment system was initially rented so that it could be evaluated for up to a year before making the capitol investment to purchase the equipment.

The Treatment System

In April 2001, Lakeside Industries mobilized a sand filter and other necessary equipment to evaluate the treatment efficiency of chitosan-enhanced sand filtration for the treatment of stormwater and process water prior to surface water discharge. Prior to equipment mobilization, bench scale treatability studies were conducted to determine treatment effectiveness, anticipated dose rates, and expected contaminant removal rates. The bench-scale tests showed an unusually high degree of contaminant removal.

The equipment installed included a Baker Tanks electric-powered 6-inch submersible pump, a Baker Tanks 54"-diameter, 4-pod sand filter (#30 silica sand), a Baker Tanks metering pump, and various hoses and fittings as required to connect the system (see Figure 1). The Chitosan Acetate was added to the water at the intake of the water pump. Samples of influent and effluent water were taken from sample ports on the filter module. Turbidity was measured using a Hach 2100P turbidimeter, pH with a Hach portable pH meter, specific conductance with a standard conductivity meter. An electronic data logger that is attached to the sand filter monitored sand filter backwash cycles.

The treatment system was sized and assembled by Baker Tanks, the leaser of the treatment system components. Treatment system chemistry, chemical metering, system calibrations, and water quality monitoring was performed by Natural Site Solutions (NSS). Treatment system startup was monitored by Baker Tanks and NSS to ensure that all components were functioning properly and that treatment system effluent was consistently meeting permit discharge standards.

Following the initial treatment system start-up, NSS and Baker Tanks worked together to optimize the treatment system and monitor the output to determine the effectiveness of treatment system. On May 3rd the chitosan-enhanced sand filtration system at Lakeside Sand and Gravel was activated for a full-scale test run. It was anticipated that this test would answer the following questions:

• Sand filter limitations (hydraulic and contaminant load capacities)
  
• Chitosan Acetate dose rate to sediment load relationship
  
• Filtrate quality (turbidity, conductivity, pH)
  
• Sand filter backwash requirements (duration, frequency, flow rate)
  
• Proper sand filter differential pressure settings

Full-Scale Test Results

The results of the startup test show an average reduction in turbidity of 97.6 percent with no change in pH or conductivity. A sample of effluent from the treatment system was collected and submitted to Parametrix, Inc. for evaluation of effluent toxicity to rainbow trout. The purpose of this toxicity evaluation was to demonstrate that actual site water treated with the chitosan-enhanced sand filtration was not toxic to rainbow trout. The effluent exhibited no toxicity and 100% of the test population survived the test.

With this exceptional level of performance the decision was made to put the system on line for additional monitoring. Over the following months the system was monitored for contaminant removal performance and one such monitoring event on March 1st, 2002 presents a representative snapshot of the system performance.

During the March 1st, 2002 test period lasting 2 hours 15 minutes, at a system flow rate of 650 gpm, the mean turbidity reduction was 98.6 percent with no significant change in pH or conductivity. Influent turbidities ranged from 40.1 to 161 NTU. Effluent turbidities ranged from 0.41 to 1.02 NTU. The chitosan dose rate was approximately 0.35 mg/L.
Total phosphorus was reduced by 50.6 percent from 102 mg/L to 50.4 mg/L. Arsenic, copper, lead and zinc were reduced 34.1%, 59.0%, respectively. All three aquatic toxicity tests passed with 100% survival.

Conclusions

The chitosan-enhanced sand filtration rental system performed so well that in the fall of 2003 Lakeside Industries decided to purchase a permanent system. The permanent system has been processing all of the facility's storm- and process water since. Although the facility's stormwater permit sets a benchmark for effluent turbidity at 50 NTU the treatment system consistently produces water less than 5 NTU which is in compliance with the State of Washington's surface water standards. The treated water exhibits no aquatic toxicity with the added benefit of significant phosphorus and heavy metals reductions.

"We've successfully treated all our stormwater, [Over 137,000,000 gallons], with this system." said site superintendent Pat Van Eaton.

Chitosan-enhanced sand filtration has proven to be a safe, cost-effective and easily implemented alternative as a stormwater and process water treatment technology at sand and gravel operations. As surface water discharge regulations continue to tighten nationally and as these regulations are more routinely enforced this technology can play an important future role.