A DESIGN FOR A BOAT WASHWATER TREATMENT ECOSYSTEM

Transcription

1 % 5% 10% 15% 20% 25% Loading Rate ph A DESIGN FOR A BOAT WASHWATER TREATMENT ECOSYSTEM May, P. I. (1,2)*, C. Streb (2) and P. Kangas (1) 1) Marine Estuarine Environmental Science & Ecological Engineering Program, University of Maryland, College Park, Maryland 2) Biohabitats, Inc., Baltimore, Maryland INTRODUCTION This report describes the research development of a living machine ecosystem for treating wastewater generated by pressure washing of boat bottoms in a marina on Baltimore Harbor. Included in the report are water quality measurements of wastewater and results of wastewater treatment experiments with a bench-scale model. All of the work was done at the Tidewater Marina on the southern shore of Baltimore s Inner Harbor. The ultimate goal of the project is to build a fullscale living machine for sustainable wastewater treatment based on the results reported here. WASTEWATER COMPOSITION Water quality measurements of stored boat washwater were made initially to test for possible pollution problems. The wastewater from the washing contains the remains of the organisms along with some anti-fowling paint residue from the boat bottoms. This washwater is collected by drainage and stored in an underground vault that acts as a septic tank. Solids settle out and are stored in the tank while liquids can leach out of the tank and move through the groundwater to the harbor. Pollution from this type of wastewater does not seem to have been studied before and measurements reported here may be the first time pollutants in boat washwater have been quantified. Data on water quality from initial sampling of the washwater storage vault at Tidewater Marina are shown in Tables 1 and 2. This wastewater can be characterized as having low dissolved oxygen concentrations, due to elevated levels of biochemical oxygen demand (BOD) and chemical oxygen demand (COD), and high levels of heavy metals, especially copper and zinc, in relation to EPA criteria. Nutrient concentrations (nitrogen and phosphorus) were not expected to be high in this type of wastewater and, therefore, they were not measured. Overall, the preliminary measurements made in the washwater vault suggest that discharge of this wastewater through seepage will degrade water quality in Baltimore Harbor. Also, pollutants in the washwater seem to increase through the summer as boat washing increases, as reflected by increased concentrations in water pumped from the vault during the experimental trials. Table 1. Washwater storage tank data: all data in mg/l (Enviro-Chem Laboratories, Hunt Valley, Maryland) Parameter Detection 4/11/05 4/27/05 6/1/05 6/15/05 Average EPA-CCC* Limit BOD COD Cadmium Chromium 0.01 < < Copper Lead < < Mercury < <.001 <.001 < Nickel <0.02 < < ABSTRACT An unusual wastewater is produced in marinas where boats area lifted from the water and pressure-washed to remove attached organisms from the hulls. This wastewater contains elevated concentrations of certain heavy metals, such as copper, nickel and zinc, that are associated with boat bottom paints and propeller assemblies. A design for a constructed ecosystem to treat this wastewater is described in this presentation. A living machine-type system was designed and tested at the bench-scale for treatment performance at a marina on Baltimore Harbor, Maryland. The system is a sequence of connected aquatic tanks including a recirculating peat filter, an oyster shell bar and a macrophyte tank. Heavy metals were effectively removed when the wastewater was run through the system at low loading rates (five percent turnover per day) but performance was reduced at high loading rates (20 percent turnover per day). Aspects of the design are described and plans for a full-scale system are presented. BIOTA IN THE LIVING MACHINES Even though concentrations of some pollutants were very high in the wastewater, biota was found throughout the living machines. Filamentous green algae, aquatic larvae (mosquitoes and midges) and snails were common. Also pitcher plants placed in the peat filter as well as water hyacinths and duckweed in the final aquatic tanks grew vigorously until near the end of the experiments in September of These and other biota provide some of the treatment processing within the living machines and also serve as a bioassay that water quality is improved in the systems. Figure 3. Drop in ph From the Waste Holding Tank to the Peat Filter Tank for Conditions of Fresh Peat Substrate EXPERIMENTAL RESULTS All of the experiments were conducted in the same way: washwater was pumped from the storage vault into an aboveground waste tank and then loaded into the living machine at known rates. As wastewater moved through the system, treatment processes acted to remove pollutants and improve waste quality. Because of this experimental design, all data tables are arranged in a sequential fashion from the start of the system at the waste tank on the left-hand side of the tables to the end of the system at the water hyacinth tanks on the right-hand side of the tables. The expected trends are that pollutants will decrease and water quality parameters will increase from left to right in the data tables. Throughout most of the summer the experimental results were positive and consistent. However, at the end of the study in September the peat filter clogged and malfunctioned, causing untreated wastewater to pass through the system. WATER QUALITY RESULTS Water quality data are given in Table 2. The most important patterns are for dissolved oxygen concentration and ph. Dissolved oxygen increased through the system due to physical aeration and photosynthesis by aquatic plants. ph decreased initially due to the acidifying properties of the recirculating peat filter, but then increased through the rest of the system, as the acidity was neutralized. The ph pattern reflects the net cation exchange capacity of the living machine and it may be a good indicator of overall treatment system function. Table 2. Water quality data 6/21/05 Parameter Tank 0 Tank 1 Tank 2 Tank 3 Tank 4 waste tank peat filter aquatic tank oyster shell hyacinths Temperature (degrees C) Dissolved oxygen (mg/l) Oxygen saturation (%) Conductivity (microsiemens) ph Salinity (0/00) Conductivity seems to act as a tracer of wastewater flow through the system, since it is relatively unaffected by the treatment processes. Conductivity is high in the wastewater but low in the tap water used to originally fill the tanks. Thus, conductivity increases over time during the experimental runs until an equilibrium is reached with a constant high value throughout all of the aquatic tanks in the sequence. Loading rate effects were evident with patterns of dissolved oxygen and ph. The strongest treatment effects were found with the lowest loading rates and treatment decreased as loading rates increased (Figure 2). At the highest loading rates aeration was only achieved at the end of the living machines and no ph change was found as wastewater passed through the system. Silver 0.01 <0.01 <0.01 <0.01 <0.01 < Tin < < Zinc * Environmental Protection Agency Criterion Continuous Concentration (indefinite exposure to organisms) LIVING MACHINE DESIGN A living machine was designed to treat the boat washwater using the ecological engineering approach. A prototype of a full-scale treatment system was built, consisting of a series of aquatic ecosystems in connected plastic containers (Figure 1). This system was tested at the Tidewater Marina throughout the summer of 2005 for treatment capacity. In the system wastewater is first pumped from the underground washwater collection vault to a 55 gallon reservoir. A daily dose of raw washwater is pumped into a recirculating peat trickling filter and then passes through three aquatic tanks before discharge as overflow into a gravel bed. Treatment takes place through physical-chemical processes and biological metabolism as the wastewater moves through the system. Design for wastewater involves: 1) choice of ecosystem units and their position within the sequential flow through system and; 2) hydraulic dimensions of storage volumes and loading rates in order to achieve desired retention times. The dominance of heavy metals in the wastewater dictated the choice of ecosystems in the design. As the initial stage in the treatment system, a recirculating peat filter was designed. This unit is meant to simulate some aspects of a natural peat bog since peat is known to adsorb heavy metals through cation exchange mechanisms. One consequence of this high cation exchange process is a reduction in ph as water passes through the peat filter. The second aquatic tank acted solely to increase retention time in the system, but in the scaled up version floating aquatic plants will be added for additional treatment capacity. The third tank is filled with empty oyster shells which serve to increase ph of the water through solution of calcium carbonate in the shells as well as provide surface area for growth of algae and invertebrates. The fourth tank is filled with floating aquatic plants. A constant recirculating pump returns water from the fourth to the third tank to provide aeration. As wastewater is pulsed through the system, initially by a dosing pump from the peat filtrate tank and then by gravity flow, the final tank of more floating aquatics and then discharge to a gravel bed completes the process. The system was first built and tested at the University of Maryland in the early spring of The change in ph of tap water moving though the system demonstrated the physical-chemical function of the peat filter in decreasing ph and the oyster shell tank in increasing ph. During the summer of 2005, the system was moved to the Tidewater Marina and seeded with biota to add biological function for testing of treatment capacity with the addition of washwater from the underground holding vault. A single system was first tested and later two more identical systems were added to explore effects of different loading rates. The first system was operated in June-July and it had a loading rate of 2.8 gallons/day (5% per day of system volume). Afterwards, three identical systems were operated during August-September and they had loading rates of 5.7 gallons/day (low flow - 10% per day of system volume), 8.5 gallons/day (medium flow - 15% per day of system volume) and 10.8 gallons/day (high flow - 20% per day of system volume). SUMMARY Fig. 1: Tidewater Marina Heavy Metal Removal Machine Daily Sump Dose Daily Drum Dose 55 gal Reservoir Underground Boat Wash-Down Vault and Sump Pitcher Plants Peat Bed Peat Filtrate Recirculation (pulsed 4x/day) Peat Filtrate Daily Peat Dose Oyster Shell/Algae Bed 15 gal Tub 1) Boat washwater does contain high levels of certain pollutants, especially several heavy metals. Constant Recirculation Hyacinth Pump Flow Duckweed Gravel Lot Infiltration Gravity Flow 2) A bench-scale living machine was designed and tested under field conditions during the summer of 2005 for the treatment of boat washwater. 3) Water quality was improved by treating boat washwater in the bench-scale living machine, including increased dissolved oxygen concentrations and reduced concentrations of BOD, COD and heavy metals. 4) Treatment and water quality improvement were inversely related to loading rate of wastewater additions. 5) Biota, including aquatic plants and animals, were found throughout the living machines and these organisms provided some of the treatment processing. HEAVY METAL TREATMENT Data on heavy metal concentrations are given in Table 3. This table shows changes in concentrations through the living machine and total percent removal in the final column. Most of the metals were not found in high concentrations in the wastewater and no distinctive patterns were found across the living machine experiments. Good treatment was consistently found for those metals in high concentration, especially copper, zinc and nickel. Treatment effectiveness was highest at the lowest loading rates, as was found with other water quality data described earlier. Table 3. Sample heavy metal data across the medium flow treatment on 8/23/05: all data in micrograms/l Parameter Detection Tank 0 Tank 1 Tank 4 % removal* limit (waste tank) (peat tank) (hyacinths) <0.25 Cadmium >61 Chromium 5 <5 <5 <5 n/a Copper Lead <2.5 <2.5 >63 Mercury 0.2 <0.2 <0.2 <0.2 n/a Nickel Silver 2.5 <2.5 <2.5 <2.5 n/a Tin 5 <5 <5 <5 n/a Zinc * percent removal = 100-((tank 4 value/tank 0 value) x 100) BOD AND COD TREATMENT BOD and COD data in Table 4 found treatment of these parameter is consistently high, ranging from percent removal through the living machine. Interestingly, no loading rate effect was evident for BOD and COD treatment. Table 4. Treatment of BOD and COD in the living machines: all data in mg/l Parameter Tank 0 Tank 1 Tank 4 % Removal* (waste tank) (peat tank) (hyacinths) 6/21/05 BOD < COD /23/05 low flow BOD COD /23/05 medium flow BOD COD /23/05 high flow BOD COD * percent removal = 100-((tank 4 value/tank 0 value)x100)

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