污水厂脱氮无需外加碳源？高效脱氮工艺？
Current wastewater total nitrogen removal treatment practice can be significantly improved by implementation of a proven technology now offered in North America, called the DEMON®-System.
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 DEMON®-System has been successfully implemented on over 20 plants, removing more than 80 percent of the total nitrogen on this side stream. DEMON is an acronym for DEamMONnification.
DEMON工艺在欧洲已经有20余年成功的工程实践，尤其对于污泥处理段排放的富氮上清液的处理，可以获得80%的去除效率，而无需碳源投加。在中国，这部分Rejectwater大部分直接回流到污水厂的进水泵房，而再度进入后续的生化处理系统，而实际上，这部分TN负荷最高可以达到40%的污水厂进水TN负荷。
图1 DEMON脱氮工艺原理示意
It is a highly 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:
the DEMON 工艺的技术优势！
a) 40 % 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;
减少40%的能耗！
b) Zero Chemical Requirement - 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;
无需投加化学药剂；
c) 90% 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;
减少90%的污泥产量；
d) Carbon Dioxide Fixation - low carbon footprint. DEMON will fix approximately 0.4 tons of CO2 per ton of nitrogen removed, versus conventional systems which will have more than 4.7 tons of CO2 emissions per ton of nitrogen removed.
绿色可持续的技术，低的碳足迹；
DEMON system at Apeldoorn WWTP
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% of the overall wastewater load. DEamMONification of this high-strength liquor efficiently reduces the side stream nitrogen load by greater than 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.
DEMON-System - Shortcutting the Nitrogen Cycle
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%.
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.
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).
