Denitrification Systems: Even properly functioning conventioanl systems are not effective at removing nitrogen. In areas where nitrogen is a problem pollutant, existing conventional systems should be retrofitted to provide for nitrogen removal through effective linking of aerobic and anaerobic transformation processes. Systems such as sand filters and constructed wetlands (see Wetlands, Constructed below) have been shown to remove over 50 percent of the total nitrogen from septic tank effluent (USEPA, 1993). Denitrification systems are most effective when used as part of a BMP system which involves source reduction through elimination of garbage disposals and use of low-volume plumbing fixtures.
Floating Aquatic Plant (Aquaculture) Systems: Constructed shallow (generally < 3 ft.) pond systems using floating aquatic plants in the treatment of industrial or domestic wastewater. Wastewater is treated principally by bacterial metabolism and physical sedimentation. The plants take up nutrients through their roots but perform little actual treatment themselves, serving instead as an excellent substrate for microbial biomass which provides significant treatment (Reed et al., 1987). The water hyacinth Eichornia crassipes has been studied extensively for use in these systems. The major advantages are their extensive root systems and rapid growth rate. Their major limiting feature is cold temperature sensitivity, confining its use to the southern states. Other species, such as pennywort (Hydrocotyle umbellata) and duckweed (Lemna spp., Spirodela spp., Wolffia spp.), have greater cold tolerances than hyacinths and have also been used in these systems (USEPA, 1988). These systems can provide effective secondary wastewater treatment or nutrient removal, depending on organic loading rate. They have been used most often for either removing algae from oxidation pond effluents or for nutrient removal following secondary treatment. The predominant mechanism for nitrogen removal is nitrification-denitrification, while phosphorus is removed through plant uptake, microbial immobilization into detritus plant tissue, and retention by sediments. Nitrogen and phosphorus removal by the plants is achieved only with frequent harvesting. Periodic removal of accumulated sludge is required. Where anaerobically generated hydrogen sulfide odor and mosquito breeding are problematic, design modifications such as step-feeding of inflows, recycling of effluent, supplemental aeration, and frequent harvesting of plants are effective. Aquatic plant treatment systems are most effective as part of a BMP system in which they perform the role of secondary, advanced secondary, or tertiary wastewater treatment (USEPA, 1988).
Upgrade or Replacement of Failing Systems: Replacement of old, inadequate systems and repair of failing ones is an integral part of an onsite wastewater management program. Common repairs include refitting the onsite system with new inflows and outlets, creating an alternative drainfield, or the use of other alternative technologies. Replacement of the entire system may be required where the original one was inadequate, improperly constructed or installed, or where the system does not respond to corrective measures.
Local governments and other programs can facilitate remedial measures on an ongoing basis by providing technical assistance to owners, an approved roster of repair professionals, a complaint response system, and financial assistance to low income households for performing the necessary repairs (Gordon, 1989).
A number of altenative technologies are available for upgrading or replacing a failing system Gordon, 1989; USEPA, 1993). These include mound or fill systems, sand filters, and pressure distribution systems. Descriptions of these alternatives are given below. Upgrading or replacement is more effective when used as part of a BMP system which involves source reduction through elimination of garbage disposals and use of low-volume plumbing fixtures (Jarrett et al., 1985).
Alternating Bed Systems: Improper function is usually associated with the soil absorption field. The most common reason for failure of the absorption field is hydraulic overload. One retrofitting option involves construction of a backup absorption field, with the ability to route tank water to either field. The backup field is used while the primary field is rested and allowed to recover through biological activity. Fields are alternated every 6 months.
Mound (Fill) Systems: This is the most widely used alternative in some areas (Gordon, 1989), and involves the use of sand or other material to create an artificial drain field when the original soil is inadequate. Effluent flows from the existing septic tank to a pump tank, from which it is pressure-distributed uniformly up into perforated pipes embedded in the fill, which is mounded above the original soil. The mounded soil serves as the absorption field.
Pressure Distribution (Low Pressure Pipe) Systems: A storage tank and pump can be installed after the septic tank to more evenly distribute the septic tank effluent. More even distribution results in better treatment than the conventional gravity distribution method for a retrofitted system or the same treatment within a shallower soil for a new system.
Sand Filters: Several types of sand filters exist. Like fill systems, the sand filter takes effluent from an existing septic tank. In the intermittent sand filter, septic tank effluent is intermittently applied to the top of a sand bed, collected by underdrains at the bottom of the bed, and piped into a soil absorption field. In the recirculating sand filter, a portion of the sand filter effluent is recirculated to achieve more treatment, and the sand is replaced on a periodic basis (Gordon, 1989).
Wetlands, Constructed: Interest has steadily increased in the United States over the last two decades in the use of natural physical, biological, and chemical aquatic processes for the treatment of polluted waters. This interest has been driven by growing recognition of the natural treatment functions performed by wetlands and aquatic plants, by the escalating costs of conventional treatment methods, and by a growing appreciation for the potential ancillary benefits provided by such systems. Aquatic treatment systems have been divided into natural wetlands, constructed wetlands, and aquatic plant systems (USEPA, 1988). Of the three types, constructed wetlands have received the greatest attention for treatment of point source pollution. Constructed wetlands are a subset of created wetlands designed and developed specifically for water treatment (Fields, 1993). They have been further defined as:
engineered systems designed to simulate natural wetlands to exploit the water purification functional value for human use and benefits. Constructed wetlands consist of former upland environments that have been modified to create poorly drained soils and wetlands flora and fauna for the primary purpose of contaminant or pollutant removal from wastewaters or runoff (Hammer, 1992).
Constructed wetlands as defined here are not typically intended to replace all of the functions of natural wetlands, but to serve as do other water quality BMPs to minimize point source and nonpoint source pollution prior to its entry into streams, natural wetlands, and other receiving waters. Constructed wetlands which are meant to provide habitat, water quantity, aesthetic and other functions as well as water quality functions (termed created, restored, or mitigation wetlands (Hammer, 1994)) typically call for different design considerations than those used solely for water quality improvement, and such systems are not addressed here. In fact, debate continues over the advisability of intentionally combining primary pollution control and habitat functions in the same constructed facilities. Nonetheless, constructed wetlands can provide many of the water quality improvement functions of natural wetlands with the advantage of control over location, design, and management to optimize those functions. While costs can vary significantly, constructed wetlands have successfully provided these functions at lower cost than conventional wastewater treatment options. They do, however, typically require significantly more land than conventional wastewater treatment facilities. The major costs are associated with pre-treatment, pumping and transmission of water to the site, distribution within the site, earthwork, possible impermeable liner, and land costs (USEPA, 1988).
Constructed wetlands vary in their pollutant removal capabilities, but can effectively remove a number of contaminants (Bastian and Hammer, 1993; Bingham, 1994; Brix, 1993; Corbitt and Bowen, 1994; USEPA, 1993). Among the most important removal processes are the purely physical processes of sedimentation via reduced velocities and filtration by hydrophytic vegetation. These processes account for the strong removal rates for suspended solids, the particulate fraction of organic matter (particulate BOD), and sediment-attached nutrients and metals. Oils and greases are effectively removed through impoundment, photodegradation, and microbial action. Similarly, pathogens show good removal rates in constructed wetlands via sedimentation and filtration, natural die-off, and UV degradation. Dissolved constituents such as soluble organic matter, ammonia and ortho-phosphorus tend to have lower removal rates. Soluble organic matter is largely degraded aerobically by bacteria in the water column, plant-attached algal and bacterial associations, and microbes at the sediment surface. Ammonia is removed largely through microbial nitrification(aerobic)-denitrification(anaerobic), plant uptake, and volatilization, while nitrate is removed largely through denitrification and plant uptake. In both cases, denitrification is typically the primary removal mechanism. The microbial degradation processes are relatively slow, particularly the anaerobic steps, and require longer residence times, a factor which contributes to the more variable performance of constructed wetlands systems for these dissolved constituents. Phosphorus is removed mainly through soil sorption processes which are slow and vary based on soil composition, and through plant assimilation and subsequent burial in the litter compartment. Consequently, phosphorus removal rates are variable and typically trail behind those of nitrogen. Metals are removed largely through adsorption and complexation with organic matter. Removal rates for metals are variable, but are consistently high for lead, which is often associated with particulate matter.
Constructed wetlands are used for numerous types of wastewater treatment and for treating stormwater runoff, but their wastewater treatment roles have received by far the most study. A significant amount of research has been done in Europe and the United States on the usefulness of constructed wetlands for municipal wastewater treatment, and volumes have been produced on the subject by Hammer (1989), Moshiri (1993), and Reed et al. (1987), providing guidance on all aspects of conventional and alternative design, construction, operation, maintenance, efficiencies, and related considerations. Also, the USEPA and the Water Pollution Control Federation (WPCF) have both published design manuals which provide well-rounded basic coverage of design, performance, case studies with costs, and related issues for constructed wastewater wetlands (USEPA, 1988; WPCF, 1990). Most of these systems are used for secondary or advanced wastewater treatment following preliminary solids and sediment removal. Two types of wetlands are commonly used - surface flow, or free water surface (FWS), wetlands and subsurface flow systems (SFS).
Although many different designs have been used, FWSs typically include metered inflow through flow-diffusing inlets into basins or channels with soil bottoms, underlain by some form of seepage barrier, filled with shallow water and supporting emergent wetland vegetation. An operable control structure typically regulates water level while inflow rate, system volume and configuration, emergent plant stalks, precipitation, and evapotranspiration dictate residence time (USEPA, 1988). Important design features for wastewater treatment include dividing the wetland into segments that can be operated and drained separately, and provisions for effluent recycling to minimize costs (Weider et al., 1989).
SFSs typically include a trench or bed underlain with an impermeable layer of clay or synthetic liner. The bed contains media, typically some form of sand or gravel, which will facilitate the growth of emergent vegetation. Water is dispersed across one end of the channel and flows horizontally down the channel below the surface, contacting the media and plants' rhizosphere. The water is treated by filtration, sorption, precipitation, and microbiological degradation processes, very much like a horizontal trickling filter with the added component of emergent plant roots. Porosity of the media has a direct mathematical relationship with the microbial pollutant degradation rate (USEPA, 1988).
Constructed wetlands have also been used in treatment of industrial point source discharges. Pilot-scale wetlands have been used for polishing secondarily treated pulp mill effluent, which has variable pollutant levels but can be high in BOD, TSS, nitrogen, phosphorus, and chlorinated organics. Vegetated gravel bed systems have been used to successfully treat such effluent, showing good removal rates for BOD, TSS, and ammonia, and modest removal of phosphorus and organic nitrogen (Thut: 1989, 1993). Tettleton et al. (1993) found significant reductions in pulp mill secondary effluent TSS and total kjeldahl nitrogen levels using a pilot-scale FWS system. Such systems are expected to have little effect on color and chlorinated organics, which can be significant contaminants in pulp mill effluents (Thut, 1993; Hammer et al., 1993).
Constructed wetlands for wastewater treatment are most effective as part of a BMP system which includes pre-treatment of waste flows to reduce suspended solids and sediments, lowering BOD levels to manageable levels. Constructed wetlands can provide secondary treatment as well as nutrient removal under low loading rates, but should be followed by other means of tertiary treatment if high loading rates are anticipated.