F.R. Mahony & Associates Products

273 Weymouth St., Rockland, MA 02370

(781)982-9300 fax (781)982-1056

This innovative process for wastewater treatment is especially designed for filtration with the simultaneous removal of BOD, ammonia, suspended solids and nitrate-nitrogen. The system is a fixed film sequencing batch biological filter. The performance of the deep-bed Amphidrome® is guaranteed to produce an effluent which meets or surpasses regulatory standards.

F.R. Mahony & Associates, Inc. provides process design, equipment, technical guidance and testing of a complete Amphidrome® System:

The removal of soluble organic matter (SOM) from wastewater streams has been the major application of biochemical operations for many years. For typical domestic waste streams, which have a biodegradable chemical oxygen demand (COD) range between 50 - 4,000 mg/l, aerobic cultures of microorganisms are especially suitable. Removal occurs as microorganisms use a portion of the carbon in the waste stream as a food source, converting it to new biomass and converting the remaining into carbon dioxide (CO₂). The CO₂ is released as a gas, and the biomass is removed by sedimentation. To accomplish the removal of soluble organic a culture of heterotrophic bacteria must be maintained in suitable environmental conditions. The microorganisms are classified as heterotrophic because they derive their carbon from an organic source, such as the incoming waste stream, methanol, or ethanol.

With greater regulatory emphasis on eutrophication and its consequences, the removal of inorganic nutrients from wastewater has become a consideration in the design of wastewater treatment plants. The primary causes of eutrophication are the inorganic nutrients, nitrogen and phosphorus. In sea water and in tidal estuaries nitrogen is typically the limiting nutrient. Therefore, nitrogen discharge limits in coastal areas have been made especially stringent in recent years. Biological removal of nitrogen to very low levels is easily accomplished. Biological removal of phosphorus is also possible; however, it is more difficult and has a limit, after which, chemical removal is required.

In domestic wastewater, nitrogen is present as ammonia (NH₃) and as organic nitrogen (NH₂^-) in the form of amino groups. In the process of ammonification the organic nitrogen is released as ammonia, as the organic matter containing it undergoes biodegradation. Two groups of bacteria are responsible for converting ammonia to the innocuous form, nitrogen (N₂). The completion of this process occurs in two steps by completely different bacteria and in very different environments. In the first step, nitrifying bacteria oxidize ammonia to nitrate (NO₃^-) in a process called nitrification. The bacteria responsible for nitrification are chemolithotrophic autotrophs that are also obligate aerobes requiring an aerobic environment. Chemolithotrophic bacteria obtain energy from the oxidation of inorganic compounds, which in the nitrogen cycle are ammonia (NH₃) and nitrate (NO₃^-). Autotrophic bacteria obtain their carbon source from inorganic carbon such as carbon dioxide. In the second step (denitrification) facultative heterotrophic bacteria convert nitrate to nitrogen gas which is released to the atmosphere. This is accomplished only in an anoxic environment in which the bacteria use NO₃^- as an electron acceptor. The ultimate electron acceptor is nitrogen, which undergoes a stepwise conversion from an oxidation state of +5 in NO₃^- to 0 in N₂. This process may be carried on by some of the same facultative heterotrophic bacteria that oxidize the soluble organic matter under aerobic conditions. However, the presence of any dissolved oxygen will inhibit denitrification since the preferential path for electron transfer is to oxygen instead of to nitrate.

Since biological removal of nitrogen is both possible and economically viable, many of today’s wastewater treatment plants require the removal of both soluble organic matter and nitrogen. To achieve this requires: a heterotrophic population of bacteria operating in an aerobic environment to remove the SOM; a chemolithotrophic autotrophic population of bacteria also operating in an aerobic environment to convert the ammonia to nitrate, and finally a facultative heterotrophic population of bacteria to convert nitrate to nitrogen gas but in an anoxic environment. Therefore, typical treatment plant designs approach the removal of organics and nutrients in one of two ways. The first method is to combine the aerobic steps ( SOM removal and nitrification) into one operation and design the anoxic denitrification process as a separate unit operation. The second method is to design three separate unit operations for each step. The type of technology utilized greatly influences the number of unit operations required to reach the desired effluent treatment level.

Biochemical operations have been classified according to the bioreactor type because the completeness of the biochemical transformation is influenced by the physical configuration of the reactor. Bioreactors fall into two categories depending on how the biological culture is maintained within suspended growth or attached growth (also called fixed film). In a suspended growth reactor the biomass is suspended in the liquid being treated. In a fixed film reactor the biomass attaches itself to a fixed media in the reactor and the wastewater flows over it. Examples of suspended growth reactors include activated sludge and lagoons. Examples of attached growth include rotating biological contactors (RBCs), trickling filters, and submerged attached growth bioreactors, (SAGBs), also called biological aerated filters (BAFs). Extensive research has been conducted on both the activated sludge process and the RBC process, but to a lesser degree on the other types.

During the last twenty years different configurations of SAGBs have been conceived, and modest advances in the understanding of the systems have been made. The advantages of SAGBs or BAFs are that they may operate without a solids separation unit process after biological treatment, and they operate with high concentrations of viable biomass. Removal of sludge is usually achieved by backwashing the filter. In such bioreactors the hydraulic retention time (HRT) is less then the minimum solids retention time (SRT) required for microbial growth on the substrates provided. This means that the growth of suspended microorganisms is minimized, and the growth of attached microorganisms is maximized. The low hydraulic retention time results in a significantly smaller required volume to treat a given waste stream than would be achieved with either a different fixed film reactor or a suspended growth reactor for the same waste stream.

The Amphidrome® system is a submerged attached growth bioreactor process, designed around a deep-bed sand filter. It is specifically designed for the simultaneous removal of soluble organic matter, nitrogen and suspended solids within a single reactor. Since it removes nitrogen, it may also be considered a biological nutrient removal (BNR) process.

To achieve simultaneous oxidation of soluble material, nitrification, and denitrification in a single reactor, the process must provide aerobic and anoxic environments for the two different populations of microorganisms. The Amphidrome® system utilizes two tanks and one submerged attached growth bioreactor, called the Amphidrome® reactor. The first tank, the anoxic/equalization tank, is where the raw wastewater enters the system. The tank has an equalization section, a settling zone, and a sludge storage section. It serves as a primary clarifier before the Amphidrome® reactor.

This Amphidrome® reactor consists of the following four items: underdrain, support gravel, filter media, and backwash trough. The underdrain, constructed of stainless steel, is located at the bottom of the reactor. It provides support for the media and even distribution of air and water into the reactor. The underdrain has a manifold and laterals to distribute the air evenly over the entire filter bottom. The design allows for both the air and water to be delivered simultaneously--or separately--via individual pathways to the bottom of the reactor. As the air flows up through the media, the bubbles are sheared by the sand, producing finer bubbles as they rise through the filter. On top of the underdrain is 18” (five layers) of four different sizes of gravel. Above the gravel is a deep bed of coarse, round silica sand media. The media functions as filter, significantly reducing suspended solids and provides the surface area for which an attached growth biomass can be maintained.

To achieve the two different environments required for the simultaneous removal of soluble organics and nitrogen, aeration of the reactor is intermittent rather than continuous. Depending on the strength and the volume of the wastewater, a typical aeration scheme may be three to five minutes of air and ten to fifteen minutes without air. Concurrently, return cycles are scheduled every hour, regardless of the aeration sequence. During a return, water from the clear well is pumped back through the filter and overflows into the trough. A check valve in the influent line prevents the flow from returning to the anoxic/equalization tank via that route. The trough is set at a fixed height above both the media and the influent line, and the flow is by gravity back to the front of the anoxic/equalization tank.

The cyclical forward and reverse flow of the waste stream and the intermittent aeration of the filter achieve the required hydraulic retention time and create the necessary aerobic and anoxic conditions to achieve the required level of treatment.

The following equations describe the biochemical reactions that are occurring simultaneously within the Amphidrome® reactor.

The reactions governing the removal of soluble matter and ammonification are as follows:

1) COHNS + O₂ + nutrients » CO₂ + NH₃ + C₅H₇O₂N + other end products

Equation 1 accounts for the biodegradation of organic material, including ammonification, and cell synthesis. Equation 2 represents the endogenous respiration of the biomass. The carbon source for cell synthesis is provided from an organic compound; therefore, the bacteria are heterotrophic. The equations also indicate that oxygen is required for both reactions to occur.

Nitrifying bacteria are chemolithotrophic autotrophic microorganisms that obtain their energy from the oxidation of ammonia and nitrite and their carbon source from carbon dioxide. Below are the two equations for nitrification.

3) 55NH₄⁺ + 76O₂ + 109HCO₃^- Þ C₅H₇O₂N + 54NO₂^- + 57H₂O + 104H₂CO₃

4) 400NO₂^- + NH₄⁺ + 4H₂CO₃ + HCO₃^- + 195O₂ Þ C₅H₇O₂N + 3H₂O + 400NO₃^-

Equation 3 describes the oxidation of ammonia to nitrite by the bacteria Nitrosomonas. Equation 4 describes the oxidation of nitrite to nitrate by the bacteria Nitrobacter. Both steps must occur in an aerobic environment.

The final step in the removal of nitrogen from the waste stream occurs when the nitrates produced in the nitrification process are converted to nitrogen gas by the process of denitrification, described below:

The above equations show methanol as the organic carbon source; however, any organic carbon source could be used. The Amphidrome® process is designed to use the organic carbon in the waste stream, by returning nitrified effluent back to the anoxic/equalization tank, to mix with the influent. Methanol is used here for pedagogical reasons. Equation 5 is an energy reaction in which nitrate is converted to nitrite. Equation 6 is also an energy equation for which nitrite is converted to nitrogen gas. The overall reaction is shown in Equation 7.

This Amphidrome® process is designed to achieve the above reactions simultaneously within one reactor. While maintaining an aerobic environment within the filter, reactions 1-4 are promoted. The purpose of returning nitrified effluent back to the anoxic/equalization tank is to mix the nitrates with both the raw organic carbon in the influent, and any organic carbon that has been released from the stored sludge as solute. Allowing the filter environment to become anoxic will promote the reactions of Equation 7 (denitrification).

Small municipal and institutional wastewater treatment plants should not be scaled down versions of large treatment systems requiring a full-time staff of experts for process control and maintenance. While small POTWs effluent discharge permits often mandate advanced standards of treatment--including removal of N (nitrogen) and P (phosphorous) - these facilities must be capable of prescribed effluents with routine service and minimal process adjustment.

While anyone with serious experience in wastewater treatment cringes when hearing "the plant will run itself," there are combinations of processes and equipment that require only routine, nominal attention. A properly conceived and constructed RBC (Rotating Biological Contactor) plant meets the prescription. It offers high levels of treatment; is a versatile, yet stable, operation; needs minimal process attention; and has low energy requirements.