Total Nitrogen Removal without Carbon

Tue, 09/20/2011 - 12:32pm
CHANDLER JOHNSON, Chief Technology Officer, World Water Works Inc.

World Water Works 2Wastewater treatment facilities use various treatment processes to provide safe effluent levels before discharging into waterways. These systems are selected based on several criteria, including the quality of water entering the treatment plant, the quality of finished water desired, the capital investment required, ongoing operational costs, flexibility and longevity.

For many wastewater facilities, meeting progressively stricter Environmental Protection Agency (EPA) and State Regulatory Agency water quality regulations requires major facility upgrades. In many areas, the tightening regulations are focused on total nitrogen removal. Nitrogen removal is an environmentally significant process. Negative environmental effects from the release of ammonia into the environment include the depletion of oxygen in watercourses and lakes, and the release of nitric oxide and nitrous oxide, which are important greenhouse gases, hence the continuing tightening regulations on nitrogen levels by the EPA.

Total nitrogen removal has become one of the most significant cost factors a wastewater facility faces. To comply with the regulations, facilities are confronted with major plant upgrades that include nitrification and denitrification. These systems typically require significant space, substantial capital upgrades, and impact both energy and chemical operational costs.

An analysis of the mass balance of a wastewater treatment plant reveals that up to 40 percent of the nitrogen load into the plant can come from the dewatering pressate or centrate stream return line. There is a direct relation between the efficiency of the wastewater solids digestion process and the release of ammonia. This effect is visible in the ammonia concentrations of liquors produced in the dewatering of digested biosolids.

The pressate or centrate from these dewatered solids is returned to the head of the plant. Treating this side stream then can have significant advantages with tremendous overall economic impact.

In Europe, a process called the DEMONR-System has been successfully implemented on more than 20 plants removing more than 80 percent of the total nitrogen on this side stream. DEMON is an acronym for DEamMONnification. It is a cost-effective technology for the total removal of nitrogen compounds from wastewater with high concentrations of ammonia. The technology is based on a biological process of partial nitritation and autotrophic nitrite reduction. The process was developed and patented by the University of Innsbruck, Austria, and has evolved into a proven technology throughout Europe. DEMON has recently been brought to the North American market by World Water Works Inc.

This DEMON process can play an important role in plant-wide efforts towards energy self-sufficiency, reduced costs and optimized footprint of any wastewater facility. Compared to other biological processes for total nitrogen removal, the DEMON process is characterized by:

  1. A 40 percent reduction in energy — only a portion of ammonia is oxidized to nitrite resulting in using 40 percent of the oxygen required compared to traditional nitrification processes;
  2. Zero chemical requirements — the denitrification process is completely bypassed in the DEMON process, eliminating the need for a carbon source. This savings alone can often yield less than a five-year return on the capital investment;
  3. A 90 percent reduction in sludge production — because no external carbon source is used for conversion of the nitrite to nitrogen gas, there is a low yield of deammonifying bacteria resulting in 90 percent less sludge production;
  4. Carbon dioxide fixation — a low-carbon footprint. DEMON will fix approximately 0.4 tons of CO2 per ton of nitrogen removed versus conventional systems, which will have greater than 4.7 tons of CO2 emissions per ton of nitrogen removed.

In an optimized process scheme of a traditional wastewater treatment plant, most of the biosolids from primary and secondary clarifiers are transferred from the liquid train to sludge digesters to generate methane and energy. Ammonia gets released from anaerobic solids digestion and represents a nitrogen-return load of approximately 15 to 40 percent of the overall wastewater load. DEamMONification of this high-strength liquor efficiently reduces the side stream nitrogen load by greater then 80 percent. Case studies demonstrate the feasibility of energy self-sufficiency of a wastewater treatment plant using this system. In other words, it is feasible that a wastewater facility can not only operate energy neutral, but even produce enough energy to sell back to the grid with the implementation of this technology on both the side stream and main stream.

Traditional Wastewater Nitrogen Removal

The removal of nitrogen is affected through the biological oxidation of nitrogen from ammonia to nitrate (nitrification), followed by denitrification, the reduction of nitrate to nitrogen gas. Nitrogen gas is released to the atmosphere and thus removed from the water. Traditional wastewater nitrification/denitrification requires large amounts of energy and carbon to obtain low effluent nitrogen limits. Alkalinity is sometimes required to maintain an efficient system, while extra sludge is produced due to the use of an external carbon source. Operational dissolved oxygen levels range from 1.0 to 2 mg/L.

Nitrification is the process by which ammonium (NH4+) or ammonia (NH3) is oxidized into nitrite (NO2-) by ammonia-oxidizing bacteria (AOB), often Nitrosomonas spp, and the NO2- is further oxidized into nitrate (NO3-) by nitrite-oxidizing bacteria (NOB), often Nitrobacter spp. Within the two processes of nitrification — nitritation and nitratation — the bacterial groups are both chemo-litho-autotrophic, meaning their only energy source is chemical energy. Their electron-donor is an inorganic compound and their carbon source is carbon dioxide (CO2), or functionally bicarbonate (HCO3-).

DEMON-System — Shortcutting the Nitrogen Cycle

World Water WorksThe DEMON-System provides the maximum possible shortcut of the traditional nitrification /denitrification process. It involves two process steps — the partial nitritation of ammonia and the subsequent anoxic oxidation of the residual ammonia and nitrite to nitrogen gas. About half the amount of ammonia is oxidized to nitrite, and then residual ammonia and nitrite is anoxically transformed to elementary nitrogen. The total nitrogen removal is accomplished using a stochiomeric oxygen demand of only 40 percent. Both process steps are catalyzed by different groups of organisms: a population of aerobic autotrophic ammonia oxidizers, and a consortium of anaerobic autotrophic ammonia oxidizers (anammox) whose intensely red color is typical.

By reducing the amount of ammonia being converted to nitrite, only 40 percent of the energy used by conventional nitrification is required. Additionally, no external carbon source (methanol) is needed due to the autotrophic nature of the process. The method technically is performed in a sequencing batch reactor (SBR) plant in which the individual steps take place in a timely sequence. The reactor is first gradually filled with centrate and the content is alternately aerated and mixed. Nitritation occurs during aerated periods and deammonification occurs during anoxic/anaerobic periods.

At the end of the fill and aerate phase, both aeration and mixing are stopped and the sludge blanket is allowed to settle. The clear supernatant is then discharged from the reactor. As the deammonifying bacteria have optimal settling characteristics, the discharge system is simple. At the end of the discharge phase, the DEMON reactor is ready for the next cycle.

The DEMON sludge forms great dense pellets (1,010 cells per ml). The growth rate of this sludge is very low, requiring a mandatory high sludge retention time. Although this sludge has a slow growth rate, it is quite resilient. For wastewater treatment plants newly starting with the DEMON process, a concentrated quantity of anammox sludge is provided to accelerate the startup process.

Process Controls

DEMON is designed as a fully automated process with a patented control strategy. Finely-tuned process controls are critical to closely monitor operating parameters, and maintain consistent and high-quality effluent conditions. Operator participation is limited to adjustment of sensors for pH, oxygen, ammonia and volume of sludge. No chemicals are being added.

The control system is based on minute variations in pH, resulting in a very simple and stable process operation. The established bandwidth for pH fluctuation is approximately 0.1 pH units. During the fill and aerate phase, the reactor is alternately aerated to convert ammonia into nitrite, which causes the pH to drop. When aeration is stopped, the pH will rise again. The aeration is then restarted and the cycle is repeated.

It is the relative change in pH value that is critical. Nitrite-oxidizing bacteria compete with anammox for the available nitrite, producing changes in pH that are used to monitor nitrite production. The measurement of relative changes in pH over a short time period is accurate enough to control the process.

Enrichment of the Anammox Biomass

A significant feature of the DEMON process is its patent-pending cyclone device for additional enrichment of the specialized slowly growing anammox biomass. Since anammox is predominantly aggregated in a heavy granular fraction, the cyclone-produced centrifugal forces select the anammox populations, while wasting the AOB/NOB populations, and decouple the sludge retention time (SRT) from the system's operation. The substantially higher mass of anammox in the system compensates for the slower kinetics of these organisms compared to AOBs. This surplus in retention of compact red granules enhances process robustness and treatment capacity.

By doubling the mass ratio of anammox compared to aerobic AOB, the robustness of the process against disturbances like over-aeration, temperature drop or a flush of excess organic carbon is drastically improved.  

Ammonia Removal Efficiency

The DEMON process can be successfully applied for the removal of ammonia from sludge liquors, without the need for an external carbon source or any other chemical. The use of a cyclone in the DEMON process allows for different sludge retention times (SRTs) and for different types of bacteria, thus greatly enhancing process stability.

Centrates can show large variations in ammonia concentration, which are mainly caused by different batches of biosolids being digested. Despite these variations, the DEMON process operates consistently with an ammonia removal efficiency of 85 to 92 percent. This reduction in ammonia load is a critical benefit to the main treatment process.

Because DEMON saves 60 percent of the energy consumption of a conventional plant, and the dosage of external carbon can be completely avoided, the amount of excess sludge produced is extremely small and the corresponding disposal costs are minimized.

In addition to significant savings in energy and the complete abandonment of organic carbon, the DEMON process has another very important advantage of less greenhouse gas production. While other biological processes produce large quantities of carbon dioxide (> 4.7 t CO2 / t N), the DEMON system ties carbon dioxide (-0.4 t CO2 / t N).

For industrial wastewater plant applications, the benefit is significant. In such plants, initially the carbon (BOD) is eliminated, and subsequently in a further stage, the nitrogen is removed. In the DEMON process, carbon and nitrogen can be eliminated very efficiently and economically in a single process step.

DEMON-System is designed to be technically simple and robust. And the investment costs are in the range of traditional and alternative methods, yet it has a fraction of the operational costs of these traditional systems. It is inevitable that with these cost and operational efficiencies, municipal wastewater treatment plants and industrial plants handling high-strength, high nitrogen content effluents will gravitate towards adopting this new-generation process for nitrogen removal.

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