Method for treating wastewater in urban wastewater treatment plants

Abstract

The present invention relates to a method for treating wastewater in urban wastewater treatment plants that comprises a first step in which urban wastewater is subjected to controlled aeration in a first reactor and passes to a solids settler under the effect of gravity, the water coming out of the settler due to overflowing; a second step of partial nitritation with aerobic granular sludge in a biological reactor, in which the COD (Chemical Oxygen Demand) at values below 125 ppm and the suspended solids at values below 35 ppm are removed from the wastewater, and partial nitritation of the ammonium coming out of the biological reactor with a NO27NH4+ ratio suitable for subsequent autotrophic nitrogen removal; and a third step of anaerobic oxidation of ammonium in a granular ANAMMOX biomass biological reactor.

Description

[0001] DESCRIPTION

[0002] Process for the treatment of wastewater in urban wastewater treatment plants.

[0003] Technical sector.

[0004] The present invention relates to a process applicable in wastewater engineering, and suitable for the treatment of wastewater in urban wastewater treatment plants (EDAR).

[0005] State of the art

[0006] Currently, several technologies based on activated sludge are known for the treatment of wastewater, some or others being used depending on the type of wastewater to be treated and the available space.

[0007] Below are some of the technologies currently used separately in different water treatment processes,

[0008] The high-load activated sludge process, based on bioflocculation, adsorption, and bioaccumulation of organic compounds, was developed to capture organic matter present in wastewater and minimize biological oxidation.

[0009] High-load activated sludge technology maximizes the transfer of organic matter to the sludge line to produce biogas.

[0010] Aerobic granular sludge technology is used in urban wastewater treatment to treat pretreatment or primary settling effluents and carry out conventional complete nitrification. In complete nitrification, ammonium is first converted to nitrite and then to nitrate. A disadvantage of this technology is that it requires significant amounts of energy to provide the oxygen necessary to carry out complete nitrification, so much so that aeration costs are the main energy cost in an urban wastewater treatment plant. Current treatment technologies consume significant amounts of energy to provide the oxygen necessary to carry out conventional nitrification.

[0011] Based on aerobic granular technology, the Nereda® system is well known, developed by Royal Haskoning DHV, consisting of a treatment plant with a single type of reactor - aerobic granular sludge with specific geometry and characteristics.

[0012] Also well-known is the granular biomass-based technology ANAMMOX® (autotrophic biomass that oxidizes ammonium and nitrite to nitrogen gas), developed by PAQUES and used to treat industrial water and return water from wastewater treatment plants under conditions of high loads and temperatures that are radically different from those found in the main water line of wastewater treatment plants.

[0013] Current wastewater treatment technologies do not optimize the diversion of organic matter to the sludge line to maximize biogas production.

[0014] On the other hand, attempts to incorporate ANAMMOX biomass into wastewater treatment processes in the main treatment line have not achieved process stability to date.

[0015] Current technology has only been able to develop aerobic granular sludge to directly treat wastewater inlet water, or settled water with a high concentration of organic matter that facilitates the development of granular biomass and heterotrophic nitrogen denitrification. In the proposed process, high-load activated sludge effluent, with a lower concentration of organic load, is used. By continuously and sequentially feeding different lines of sequential biological reactors (SBR) operating at constant volume and with volume exchanges between 40% and up to 70%, and through selective purging, granular sludge is developed using the outlet water from a high-load activated sludge process, and nitrification to nitrite is partially halted.

[0016] Regarding autotrophic anaerobic ammonium oxidation, it has only been developed to treat the returns from anaerobic digestion units, with high nitrogen concentrations and temperatures, and this technology is not being used in the treatment of water from the main line of a wastewater treatment plant due to the technical difficulties it entails.

[0017] To autotrophically remove nitrogen from the main line of an urban wastewater treatment plant, one must deal with low concentrations of ammonium and low temperatures in winter.

[0018] The technical challenge posed is the development of a wastewater treatment process that provides significant energy savings compared to current technology and a technical breakthrough that overcomes the difficulty of process control and enables stable autotrophic nitrogen removal from the main water line.

[0019] Explanation of the invention.

[0020] The process of the invention is based on the combination of three consecutive treatment stages that include a first high-load activated sludge reactor, an aerobic granular sludge reactor capable of achieving partial nitritation of ammonium, and a third reactor for the anaerobic oxidation of ammonium. The combination of the three reactors, in the mode of operation described herein, provides energy savings in the overall process and a technical advance since it solves the great difficulty of stable control of autotrophic nitrogen removal in the water line of a WWTP.

[0021] This process is based on the adaptation of aerobic granular sludge technology to partial nitritation to treat low-load water in the main line of a wastewater treatment plant by partially oxidizing ammonium to nitrite and obtaining an effluent suitable for the anaerobic ammonium oxidation process in the water line. The influent to the aerobic granular reactor is water from a high-load activated sludge reactor. To date, the combination of these technologies has not been implemented due to the technical difficulty involved in combining them.

[0022] The process for the treatment of wastewater in urban wastewater treatment plants, object of the invention, comprises the following stages:

[0023] - a first stage consisting of a high-load biological treatment in which the urban wastewater to be treated is kept in a first reactor for a retention time of less than 1 hour and is subjected to controlled aeration by means of an oxygen probe at a dissolved oxygen concentration of less than 1 ppm and subsequently the water passes to a decanter in which the solids settle by gravity and the water treated in said first stage exits by overflowing the decanter;

[0024] - a second stage of partial nitritation in an aerobic granular biological reactor, in which the COD (Chemical Oxygen Demand) is removed from the wastewater to values ​​below 125 ppm and the suspended solids to values ​​below 35 ppm and by controlling the exchanged volume, the age of the sludge, the length of the cycles and the concentration of dissolved oxygen during the aeration phase, a partial nitritation of the ammonium is carried out, which manages to leave the biological reactor with an NO2- / NH4+ ratio suitable for the subsequent autotrophic elimination of nitrogen and,

[0025] - a third stage of anaerobic oxidation of ammonium in an ANAMMOX biomass biological reactor, which manages to remove total nitrogen from the wastewater to values ​​below 6 ppm.

[0026] The combination of these three stages in wastewater treatment allows for significant energy savings and a technical advance in the stability and control of autotrophic nitrogen removal.

[0027] With the process of the invention, firstly the organic matter is derived with high-load activated sludge, then an aerobic granular reactor adapted to carry out partial nitritation of nitrogen is used (significant energy savings in aeration) and finally the ANAMMOX technology is used to eliminate nitrogen in a completely anaerobic and autotrophic manner (without the need for air or organic matter).

[0028] The scheme resulting from the process of the invention comprises a first stage of high-load activated sludge with high derivation of organic matter to the sludge line followed by a second stage in a reactor with aerobic granular sludge adapted to partial nitritation at low loads and low temperatures designed to create an influent suitable for finally carrying out the anaerobic oxidation of ammonium in the third stage.

[0029] The invention comprises, in the first stage, the recirculation of the settled solids to the first reactor, except for a portion that is purged from the system, maintaining a concentration of suspended solids in the first reactor below 3000 ppm. In this first stage, between 40 and 60% of the COD (Chemical Oxygen Demand) is eliminated from the wastewater.

[0030] The second stage is carried out in a reactor consisting of several lines or vessels fed from the bottom through the settled sludge bed and fed sequentially. Each vessel cycles through phases of simultaneous feed / overflow (constant level), aeration, and settling. During the feed phase, approximately 50% of the volume is exchanged. At the end of each settling phase, excess solids are purged laterally. Granular sludge and floccular sludge coexist in the vessels. A concentration of free nitrous acid (FNA) inhibits nitrite-oxidizing bacteria (NOB), so that the water exits with an ammonium / nitrite ratio close to 1.32. The retention time in this second stage is less than 14 hours.

[0031] Each line is fed through the reactor bottom via grates that ensure even distribution throughout the reactor floor. The aeration system is located below the reactor feed grate.

[0032] Each reactor line operates in daily cycles. Each cycle includes at least one feed / overflow cycle, one aeration cycle, and one settling cycle.

[0033] On the upper surface of the reactor there is a homogeneous system for collecting clarified water through channels or pipes that allow the treated water to be evacuated during the feeding phase.

[0034] Below the evacuation system is a valve or gate that allows, if necessary, the liquid level to be lowered below the evacuation system to prevent it from filling with solids during the aeration phase. The reactor has a purge system to purge the suspended biomass above the level of the settled granules. Aeration is controlled to achieve the desired ammonium concentration and / or NO27NH4+ ratio using ammonium and pH probes. Rx, nitrite, and nitrate probes can also be used to control aeration. Another advantage of the process of the invention, combining the three stages indicated, is that the growth of nitrite-oxidizing bacteria (NOB) is inhibited and favorable conditions are obtained for the third stage, ANAMMOX, which have not previously been achieved in the water line of an urban wastewater treatment plant in a stable manner.

[0035] The second stage with partial nitritation aerobic granular sludge ensures an effluent with low concentrations of solids and organic matter, which are dangerous for the ANAMMOX process, and at the same time achieves autotrophic elimination of nitrogen in two separate stages (second and third stages of this process) to minimize aeration energy consumption, organic matter needs and required volume.

[0036] The application of partial nitritation in the second stage allows saving up to 50% of oxygen for nitrogen removal compared to conventional total nitrification.

[0037] The third stage, fed with the effluent water from the second stage, is carried out in a stirred reactor with ANAMMOX granular biomass in which the bacteria eliminate nitrogen anaerobically and autotrophically and the water leaves the clarification zone meeting the following discharge quality:

[0038] The application of ANAMMOX technology in this third stage of water treatment allows the maximum amount of organic matter to be diverted to biogas production, as it is not required for conventional denitrification. The increased biogas produced can be used to generate electricity, reducing the energy consumption of the WWTP, or as biomethane with prior purification. The process of the invention, as mentioned above, is a combination of the three stages indicated and constitutes a modification of the basic process diagram of a conventional wastewater treatment plant, providing several advantages, including:

[0039] - represents a step towards the goal of reducing greenhouse gases by reducing the energy consumption of urban wastewater treatment plants, which are currently large consumers of electricity;

[0040] - allows control of the main water line process which, thanks to the concentration of free nitrous acid (FNA) obtained and together with the granules, gives stability to the partial nitritation process, allowing the autotrophic reduction of ammonium in two stages at low temperatures and low loads;

[0041] - Since it does not require secondary settling, it is a more compact solution than conventional primary settling plus biological treatment of activated sludge with nutrient removal, reducing the environmental impact associated with the construction of wastewater treatment plants and freeing up space for other uses.

[0042] In the event that it is necessary to reduce the phosphorus concentration of the effluent, the invention contemplates that the treatment process may comprise a dosage of FeCI3 in any of its stages.

[0043] In the event that the ammonium / alkalinity ratio of the raw water is too low and the level of free nitrous acid (FNA) required in the reactor to inhibit the growth of nitrite-oxidizing bacteria (NOB) is not achieved, the invention comprises increasing the ammonium / alkalinity ratio of the influent by dosing acid in the second stage.

[0044] Once the nature of the invention has been sufficiently described, it is hereby stated for the appropriate purposes that any modifications deemed appropriate may be introduced into the described process, provided that this does not entail an alteration of the essential characteristics of the invention claimed below.

Claims

CLAIMS 1. A process for the treatment of wastewater in urban wastewater treatment plants, comprising: - a first stage consisting of a high-load biological treatment in which the urban wastewater to be treated is kept in a first reactor for a retention time of less than 1 hour, subjected to controlled aeration by means of an oxygen probe at a dissolved oxygen concentration of less than 1 ppm and subsequently the water passes to a decanter in which the solids settle by gravity and the water treated in said first stage exits by overflowing the decanter; - a second stage of partial nitritation with aerobic granular sludge in a biological reactor, in which the COD (Chemical Oxygen Demand) is removed from the wastewater to values below 125 ppm and the suspended solids to values below 35 ppm and by controlling the age of the sludge and / or the length of the cycles and / or the concentration of dissolved oxygen during the aeration phase, a partial nitritation of the ammonium is carried out, which manages to leave the biological reactor with a NO27NH4+ ratio suitable for the subsequent autotrophic elimination of nitrogen and, - a third stage of anaerobic oxidation of ammonium in an ANAMMOX granular biomass biological reactor, in which Total Nitrogen is removed from the wastewater to values below 6 ppm.

2. The process for treating wastewater according to claim 1, wherein the first stage comprises recirculating the settled solids to the first reactor, except for a portion that is purged from the system, maintaining a concentration of suspended solids in the first reactor below 3000 ppm.

3. The process for treating wastewater according to any preceding claim, comprising reducing the phosphorus concentration of the effluent by dosing FeCl3 at any stage of the process.

4. The process for treating wastewater according to any preceding claim, comprising increasing the ammonium / alkalinity ratio of the influent by dosing acid in the second stage.

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