THE GOAL: “DENITRIFICATION” – Reducing Nitrogen in Our Waters
1. Typical BIOLOGICAL approach: NITRIFY then DENITRIFY
The biological approach uses naturally occurring bacteria to facilitate chemical transformations in wastewater. Most commercial systems use this approach. The National Sanitation Foundation, the leading independent organization that develops standards and tests products to protect the environment and human health, with the American National Standards Institute (“NSF/ANSI”) provide certification for proprietary systems that reduce nitrogen (NSF/ANSI 245). The United States Environmental Protection Agency (EPA) in conjunction with NSF, has also tested products through the Environmental Technology Verification (“ETV”) Program to verify the performance of commercial-ready, environmental technologies. Any system that passes NSF/ANSI 245, which requires a 50% reduction, also needs to meet NSF 40 requirements for Biological Oxygen Demand (“BOD”), Total Suspended Solids (“TSS”), pH, noise and odor limits. Suffolk County intends to approve systems that treat to below 19 mg/L for single family homes. Owners in sensitive watersheds should consider systems that reduce nitrogen (N) levels as low as possible (75-90%), preferably below the 10 mg/L guideline for drinking water.
NSF /ANSI 350 certifies Onsite Reuse Treatment Systems, for treatment levels appropriate for human contact, although local regulations will regulate use options. Reuse, whether for shallow, subsurface irrigation, or more highly treated water suitable for reuse for toilets or surface irrigation, rightfully considers wastewater as an important resource. Reuse reduces water consumption and contaminant release to groundwater.
In raw wastewater nitrogen is bound in ammonium and organic compounds. Most of the systems start with a septic tank or chamber, where solids settle and undergo primary treatment in anoxic (no air) conditions.
Because denitrification is a two-stage process, before denitrification can occur, the ammonium (NH4) needs to be nitrified. Microorganisms use oxygen to transform ammonium into nitrite (NO2) and then nitrate (NO3). The effectiveness of this stage depends upon the amount of oxygen available, the amount of oxygen being consumed, called organic biochemical oxygen demand (OBOD), and the volume of wastewater being processed compared to the system’s capacity (hydraulic loading rates).
The denitrification stage requires a source of carbon and an anoxic (without air) environment conducive for anaerobic bacteria to thrive. Nitrogen gas (N2) and oxygen are the final products. The following are some examples of technological approaches to the biological treatment of wastewater.
|Recirculating sand filters (RSFs) nitrify by filtering the effluent through sand. Denitrification occurs by either recirculating the effluent to the anoxic septic tank or to an upflow anaerobic tank with a carbon source.
|Natural systems can purify wastewater. These are basically vegetated recirculating sand filters and/or constructed subsurface wetlands.
|Trickling filter/fixed film systems nitrify wastewater by spraying it over media, which maximizes surface area for beneficial microorganisms to grow. Typically denitrification happens by recycling a portion to a septic tank or mixing tank.
|Suspended-growth / activated sludge pumps air into a tank to activate sludge and support nitrification.Denitrification usually occurs in a settling tank.
|Submerged fixed film systems provide media to increase the surface area for beneficial bacteria to grow, improving treatment. These can be used for either anoxic chambers (denitrification) or aerated chambers that bubble air up through the liquid.
|Sequencing batch reactors (SBR) alternate periods of aeration with anoxic conditions in the same chamber. Success is relative to the time retained in the states needed for nitrification and denitrification.
|Membrane bioreactors are filters with tiny pores that block dissolved solids and bacteria from being dispersed to the dispersal system. They are often part of a suspended growth system. Some systems aerate the filters to both facilitate nitrification and scour the filters to avoid clogging. Effluent quality is usually very high and thus likely to be acceptable for reuse.
|Shallow dispersal to the top foot of soil enhances treatment of wastewater by microbial activity in the root zone and the vegetative uptake of nutrients.
|“Layer Cake/Soil Based Treatment/Nitrogen reducing bioreactor” This promising, nonproprietary system can effectively reduce N levels to less than 5 mg/L. Using shallow dispersal (top 6” of soil) over a wide area, effluent filters through a sand layer, then through a layer mixed with cellulose for a carbon source, which can be in an anoxic layer (lined), an up-flow chamber, or in a loamy soil that naturally retains moisture.
2. Waste Separation
- Waste separation is a viable way of removing contaminants and nutrients from the local waste stream effectively and inexpensively.
Simply separating greywater (typically from showers, lavatories) from black water (toilets) can increase the effectiveness of primary treatment due to long residence times in the septic tank. When used seasonally, nutrients and contaminants may be removed from the local environment if pump-outs are appropriately scheduled.
- Composting toilets efficiently remove black wastewater from the local disposal system. The nutrient-rich product can possibly be harvested for fertilizer.
- Urine separation removes nutrient rich liquid from the normal waste stream. With simple pasteurization it can be used as fertilizer.
3. Other Approaches
The natural pathways for transforming wastewater can be complex and are still being studied. There are a number of products that were designed to nitrify wastewater and are now being redeveloped to provide denitrification. A promising pathway is the Anammox bacteria, which transforms ammonium and Nitrites (NO2) to nitrogen gas (N2) and Nitrate (NO3) in an anaerobic environment. A more direct solution is an anaerobic/ion exchange (AN-IX) system with ammonium adsorbed by zeolite or other compounds, creating a product that is usable for fertilizer. The media needs to be replaced roughly every six months to ensure effectiveness. We can expect advances in treatment and reuse to transform how onsite wastewater is handled.
To learn about specific technologies suitable for single-family onsite wastewater treatment click the following link: