WASTE WATER NITRIFICATION

Abstract: Nitrification of waste water is carried out in a fluid treatment plant (1). Nitrifying bacteria and oxygen are added to the waste water to break down the ammoniacal-nitrogen to nitrate via nitrite. Dinitrogen oxide (N2O) gas produced during nitrification is measured. An increase in N2O indicates that the nitrification is failing either as a result of ammonia increase, oxygen deprivation or the presence of toxic material.

Description: WASTE WATER NITRIFICATION This invention relates to a waste water nitrification method, and concerns in particular the control of such a method. Industrial and domestic waste water is treated to remove environmental pollutants. Organic matter, ammoniacal-nitrogen and other chemicals must be removed from waste water. The removal of ammoniacal-nitrogen from municipal and industrial wastewater prior to discharge is a legal requirement . Nitrification is the biological oxidation of ammonia (NH 3 ) to nitrate (N0 3 ~ ) via nitrite (N0 2 " ) . Two bacteria genera are responsible for nitrification; Ni trosomonas oxidises ammonia to nitrite and Ni trobacter oxidises this to nitrate. Nitrosomonas oxidise ammonia to nitrite as follows :- Oxidation of ammonia to nitrite Step Reaction 1 NH 3 + 0 2 +2H + + 2e " → NH 2 OH + H 2 0 Ammonia mono-oxygenase 2 NH-OH + H 2 0 → HONO + 4H + + 4e ~ Hydroxyl amine oxidoreductase 3 0 . 5O 2 + 2H + + 2e " → H 2 0 Total NH j + 1 . 50 2 → N0 2 " + H + + H 2 0 The Ni trobacter oxidise nitrite to nitrate as follows :- Oxidation of nitrite to nitrate Step Reaction 1 HONO + H 2 0 → HON0 2 + 2H + + 2e " Nitrite oxidoreductase 2 0.5O 2 + 2H + + 2e " → H 2 0 Total HONO + 0.5O 2 -» HON0 2 Nitrification is a sensitive process, the stability of which is dependent upon numerous operating conditions; especially pH, organic matter, ammonia content, dissolved oxygen and the absence of toxins . The oxidation of a kg of ammonia to nitrate requires approximately 4.3 kg of oxygen. The nitrification process has a much greater oxygen requirement than the oxidation of organic matter in waste water, with the stability of the former process particularly effected by the oxygen supply rate and dissolved oxygen (DO) concentration. Thus, any variation in the influent waste water characteristics or operating conditions that reduces the DO concentration such that the process becomes oxic (aerobic) may have a detrimental effect upon nitrification rate and efficiency and if left unresolved process failure could occur. To prevent such an outcome continuous online monitoring is necessary. Continuous measurement of DO during nitrification is often employed although the usefulness of such monitoring is limited. Low DO concentrations can be observed when oxygen supply rates are high enough to satisfy demand and low DO concentrations alone do not imply imminent process failure. Monitoring of the nitrification during wastewater treatment is mainly done by spot analysis of effluent ammonia, nitrite and nitrate by wet chemistry methods. This method of monitoring is time consuming, resource demanding and does not provide any pre-warning of a nitrification failure. More recently, biological nitrification toxicity monitors have been installed in side streams at some wastewater treatment plants. Though expensive to install, time consuming and costly to operate this method at least provides online monitoring. It has been shown that bacteria normally associated with the nitrification process produce nitrogen oxide (NO) and dinitrogen oxide (N 2 0) under oxic conditions. It has been shown that NO and N 2 0 are formed via the reduction of N0 2 " when the nitrification process is oxygen stressed and operating under oxic conditions . Under such conditions N0 2 " is used by ammonium oxidisers as the terminal electron acceptor instead of oxygen and N 2 0 formed rather than N0 3 " . Therefore, any change in influent or operating procedures that results in oxic conditions and an increase in N0 2 " can be determined by monitoring gaseous N 2 0. Reduction of nitrite to dinitrogen oxide Step Reaction 1 N0 2 " + e " +2H + → NO + H 2 0 Total N0 2 " + 2e " + 3H + → % N 2 0 + lfc H 2 0 N 2 0 production during nitrification is strongly linked to nitrification inhibition. The invention exploits this relationship; gaseous N 2 0 concentration is determined using a N 2 0 gas analyser and used to assess the stability of the nitrification process. The concentration of gaseous N 2 0 formed during nitrification is directly related to the build-up of N0 2 " in the aqueous phase during oxic conditions. The failure of the nitrification caused by the presence of toxic substances is also detected by the increased N 2 0 production. Furthermore, the rate at which gaseous N 2 0 increases is directly related to the degree of nitrification inhibition. The advantages of this invention are considerable; non-invasive online monitoring of the nitrification process in all types of biological treatment processes can be undertaken without the need for regular spot samples, wet chemistry analysis or expensive toxicity monitors. According to the invention, there is provided a method comprising nitrification and measuring N 2 0 produced. The increase in N 2 0 indicates that the nitrification is failing, whatever the cause of the inhibition of the nitrification. An increase in ammonia concentration, a decrease in oxygen supply (however caused) and the presence of a toxin affecting the nitrifying bacteria will each result in an increase in N 2 0 production. Upon detection of nitrification failure by N 2 0 detection in a commercial installation a number of different actions can be taken to ensure correction of the failure . The rate of air flow into the aeration basin can be increased to raise the DO level . The influent rate could be reduced or stopped altogether. The rate of return of the activated sludge could be increased. Further material could be added to the aeration basin or the effluent to counter the presence of any toxic compound. The effluent could be passed to a further treatment stage or it could be passed to a storage tank for returning to the nitrification works as influent. Any or all of these actions can be taken in real time, in order to correct the inhibition of the nitrification. In order that the invention may be clearly understood, and readily carried into effect, reference will now be made, by way of Example, to the accompanying drawings, in which: - Figure 1 is a schematic representation of a fluid treatment plant, Figure 2 is a schematic representation of a hood for collecting gases formed during fluid treatment, Figure 3 is a graph showing the build up of N0 2 " in the aqueous phase and the resultant gaseous N 2 0 formed during nitrification when shock loaded with ammoniacal-nitrogen, Figure 4 is a graph showing the rate of gaseous N 2 0 formed during nitrification for different shock loads of ammoniacal- nitrogen, Figure 5 is a graph showing the increase of aqueous phase N0 2 " and gaseous N 2 0 during nitrification when there is aeration failure, Figures 6a and 6b are graphs showing the effect on the nitrification process of a second Example of shock loading, Figures 7a and 7b are graphs showing the effect on the nitrification process of a third Example of shock loading, Figures 8a and 8b are graphs showing the effect of a second Example of a reduction in oxygen on the nitrification process, Figures 9a and 9b are graphs showing the effect on the nitrification process of a third Example of a reduction in oxygen, and Figures 10a and 10b are graphs showing the effect on the nitrification process of an addition of a toxic substance. The fluid treatment plant 1 of Figure 1 is for the biological treatment of domestic or industrial waste water. The nitrification takes place in the aeration basin 2. If the waste water contains organic matter that also requires biological treatment using micro-organisms different from those carrying out the nitrification then this can also be carried out in the same basin 2. Alternatively the treatment of the organic matter and the nitrification can occur in series, in separate basins. The waste water in the form of settled sewage arrives at a tank 3. From this tank 3 influent can be pumped at a controlled rate into the aeration basin 2 which contains the two different nitrifying bacteria, Ni trosomonas and Ni trobacter. Air from a compressor 4 is fed to the basin 2 to provide oxygen. The bacteria break down the ammoniacal-nitrogen present to nitrate via nitrate and the treated sewage in the form of sludge is principally passed to a settling column 5 whereby the sludge which contains substantial amounts of bacteria (and therefore called "activated sludge") is separated from the remainder which is discharged as effluent. The activated sludge is pumped through a return pipe 6 to the aeration basin 2. The basin 2 also has the facility to divert waste sludge to a tank 7 for passing directly to a drain. As shown in Figure 2, a hood 10 placed above the aeration basin 2 of the nitrification wastewater treatment unit 1 collects the gaseous products formed. The gas produced is then transferred via a pipe 11 to an N 2 0 analyser 12 for gaseous N 2 0 concentration determination. A suitable analyser would be the Horiba VIA-510 Gas Analysis Unit. The following Examples show how the production of N 2 0 varies according to variation in operating conditions. Example 1 A nitrifying microbial population with a biomass concentration of 1 (±0.05) mg TSS (total suspended solids) l "1 were cultured in a 7 litre activated sludge process maintained at a pH of 7.2 and a temperature of 25°C. The hydraulic retention time was 7 days and the solid retention time was 23 days. The process was fed ammonia as the only energy source; at a steady state the nitrogen loading was 244 (±15) mg NH 3 -N g TSS "1 d "1 . NH 3 -N refers to nitrogen in the form of ammonia. At time zero the activated sludge plant was shock loaded by adding ammonia at a concentration of 3.3 mg NH 3 -N g TSS l "1 (16.9 mg NH 3 -N in total) . Shock loading is the addition of a relatively large amount of a substance for a short time. The variations in N 2 0 concentration in the gaseous phase and the N0 2 " concentration in the aqueous phase are shown in Figure 3. Example 2 In this Example the process and operation was as detailed in Example 1. The process was subjected to ammonia shock loads over a range of concentrations and the production rate of N 2 0 from the activated sludge determined (Figure 4) . The rate of N 2 0 production was directly related to the size of the shock load at concentrations less than 1.6 NH 3 -N g TSS IN It can therefore be clearly seen from Examples 1 and 2 that a sudden increase in the ammonia content of the influent produces an immediate and corresponding increase in Ν 2 0 formed. Example 3 The same process as described in Example 1 was operated under the same conditions in this Example. However, in this Example the aeration rate was reduced from 3.7 1 min "1 to 0.3 1 min "1 which resulted in a decrease in the dissolved oxygen (DO) level from 4.5 mg l "1 to <2 mg IN The resultant increase in Ν 2 0 concentration in the gaseous phase was closely related to the build-up of N0 2 " in the aqueous phase, as shown in Figure 5. Other changes in influent characteristics or operating conditions also lead to an increase in gaseous N 2 0 formed during nitrification instability. In particular, a significant increase in the organic loading rate to a nitrification process, resulting in process instability, would also increase the formation of gaseous N 2 0. Breakthrough of organic material from a prior process in a municipal treatment facility, or an increase in the feed loading rate from an industrial effluent are common causes of instability/failure in nitrification processes, and will also lead to increased N 2 0 production. Further Examples are detailed below that were carried out using a pilot plant consisting of two lanes, each comprising a 300 1 aeration basin 2 and a 100 1 settling column 5 as shown in Figure 1. Each basin 2 was initially filled with 150 1 wastewater and 150 1 activated sludge taken from the return activated sludge (RAS) channel of a municipal wastewater treatment works (Cotton Valley Wastewater Treatment Works, Anglian Water) . Cranfield University Wastewater Treatment Works wastewater- (mean COD 391 ± 40 mg/1, mean NH 3 44.3 ± 7.5 mg/1) was supplied at 50 1/h, thus maintaining the hydraulic retention time (HRT) at 8 hours. The hydraulic retention time is the length of time that the waste water remains in the aeration basin 1. The aeration basins and settling columns were covered to minimise liquid loss by evaporation. Frost protection and insulation were provided to the aeration basins, and the settling columns insulated. The dissolved oxygen (DO) was maintained at 4.0 ± 1.7 mg/1 and the influent pH remained within 7.31-7.60 without pH control. Solids were brushed off the basin walls daily, and sludge wasted from the aeration basins at a rate to maintain a mean cell retention time (MCRT) of 12 d. Measurements of the influent and effluent chemical oxygen demand (COD) , NH 3 and suspended solids (SS) concentrations were made three times per week; mixed liquor suspended solids (MLSS) was also measured. The N 2 0 was sampled and analysed using an Horiba VIA-510 Gas Analysis Unit. This unit accurately measures the concentrations of specific components in gas mixtures using non-dispersive infrared analysis for spot or continuous measurement of components in sample gases . Particles consisting of different atoms absorb infrared in their specific wavelength bands, and gases at constant pressure absorb infrared corresponding to their concentration. Based on this principle, the VIA-510 measures infrared light of a specified wavelength passing through a cell of sample gas held under constant conditions . Changes to the quantity of N 2 0 cause changes to the quantity of light passing through the sample cell, from which gas concentrations are calculated. The gas analyser working range was 0-200 ppm, measured to one decimal place (± 1.0 ppm) when calibrated with compressed air (zero gas) and 160 ppm N 2 0 (with N 2 balance, certified 99.99% accurate). The limit of detection was 0.1 ppm. The conditions under which the following Examples were carried out are summarised in the table below. Experimental conditions Example Type Ini tial MLDO Influent NH3 Initial effluent (mg .r 1 ) (mg.l "1 ) NH 3 (mg.l "1 ) NH 3 shock (instantaneous) 2.2 36.4 1.7 NH 3 shock (15 min duration) 2.4 36.0 1.2 LDO depletion 2.4 44.6 1.2 LDO depletion 2.9 40.1 1.9 Chemical inhibition 2.2 38.1 2.3 Examples 4 and 5 - High ammonia loadings In Example 4, one of the pilot plant lanes was shock loaded by adding 2.44g NH 3 -N (dosed as 7.25g NH 3 C1 dissolved in 1000ml H-,0) directly into the aeration tank, a dose calculated to exert an instantaneous oxygen demand of 3.5 mg.l "1 on the 3001 aeration tank. Influent ammonia in the sewage feed was 36.4 mg.l -1 prior to shock loading. The air supply was kept constant during the experiment and the starting DO was 2.2 mg.l -1 . Mixed liquor DO (MLDO) and N 2 0 in the headspace were logged every 15 min, and the effluent was sampled for ammonia, nitrite and nitrate determination. The experiment proceeded for 12 hours . Example 5 employed the same ammonia load, dissolved in sewage feed and pumped into the second pilot plant lane over a period of approximately 15 minutes . The experiment continued in exactly the same way as before. The starting DO was 2.4 mg.l -1 and the ammonia in the sewage feed was 36.0 mg.l -1 . Mixed liquor DO and N 2 0 in the headspace were logged every 5 minutes for the first 2 hours and every 15 minutes thereafter, and the effluent was sampled for analysis at 15 minute intervals for the next 10 hours. A decrease of DO in the aeration tank (Figures 6a and 7a) and an increase of N 2 0 in the exhaust gas from the aeration tank (Figures 6b and 7b) followed shock loading. Increases in N0 2 -N (nitrogen in the form of nitrite) and NH 3 -N followed after a period of time, whereas N0 3 -N (nitrogen in the form of nitrate) showed no sharp increases (Figures 6b and 7b) . In Example 4, a minimum for the DO and maximum N 2 0 was reached approximately 30 minutes after the shock load, after which DO increased and N 2 0 decreased. Both DO and N 2 0 had recovered after approximately 105 minutes. Effluent N0 2 -N (Figure 6a) and NH 3 -N (Figure 6b) remained stable at 0.7 ± 0.1 and 1.5 ± 1.4 mg.l -1 respectively, until 7 hours after the shock load was applied. N0 2 -N had recovered after a further 2.5 hours whereas NH 3 -N values remained high for the remainder of the test. An effluent sample taken 24 hours after shock loading contained 1.8 mg.l "1 NH 3 -N, indicating that the system had recovered. The observed peak in exhaust gas N 2 0 preceded the appearance of ammonia and nitrite in the plant effluent by approximately 7 hours (i.e. 0.9 HRT) . Measurement of N 2 0 gas gives a much earlier warning of nitrification failure when compared with other known methods . Ammonia loading in Example 5 produced a broader peak of N 2 0 between 15 and 35 minutes after the dose was applied. The trough in mixed liquor dissolved oxygen (MLDO) was also broader, occurring between 30 and 75 minutes after dosing. Effluent N0 2 -N (Figure 7a) and NH 3 -N (Figure 7b) increased 7 hours and 30 minutes after the shock load was applied. N0 2 -N had recovered after a further hour whereas NH 3 -N values remained high for the remainder of the test . An effluent sample taken 15 hours after shock loading contained 2.1 mg.l "1 NH 3 -N, indicating that the system had recovered. The observed peak in exhaust gas N 2 0 preceded the appearance of ammonia and nitrite in the plant effluent by approximately 6 hours and fifteen minutes (i.e. 0.78 x HRT). Examples 6 and 7 Oxygen depletion During Examples 6 and 7, aeration was reduced to maintain DO concentrations of <1.0 mg.l -1 for 1 hour. The air regulators available read from 0 to 10 bar and therefore did not give a reliable pressure reading at the low flows required, hence air flow rate measurements were not available. Air flow was altered to achieve target mixed liquor dissolved oxygen (MLDO) values rather than specified air flow rates. A submersible pump was placed in the aeration tank to maintain mixing of the mixed liquor suspended solids (MLSS) while the air supply was reduced. Headspace N 2 0, MLDO and effluent N0 3 -N, N0 2 -N and NH 3 -N were measured as in Example 5. The initial MLDO reading was 2.4 mg.l "1 and feed NH 3 -N was 44.6 mg.l "1 in Example 6, and 2.9 mg. I "1 and 40.1 mg.l "1 respectively in Example 7. Maintaining MLDO of <1.0 mg.l "1 for 1 hour led to a sharp increase in headspace N 2 0 occurring, peak values being observed one hour after MLDO fell to <1.0 mg.l "1 (Figures 8a and 9a) and followed by a decrease to pre-test levels after a further 3 - 3.5 hours. Mixed liquor DO returned to pre-test concentrations within 15 minutes. N0 2 -N and NH 3 -N were produced in the effluent 7.5 hours (Figures 8a and 9a) after the N 2 0 in the exhaust gas began to increase . Measurement of N 2 0 gas again will be seen to produce the earliest warning of nitrification failure. Minimum values for the MLDO and maximum N 2 0 were reached approximately 65 minutes after the air supply was reduced in both experiments, after which DO increased and N 2 0 decreased. Mixed liquor DO recovered approximately 20 minutes after air supply was resumed and N 2 0 after 2.5 hours in Example 6 and 3.25 hours in Example 7. Effluent N0 2 -N (Figures 8a and 9a) and NH 3 -N (Figures 8b and 9b) remained stable until 7.5 hours (0.90 HRT) after the N 2 0 detected began to increase. Effluent N0 2 -N and NH 3 -N values remained high for the remainder of the test . An effluent sample taken 12 hours after Example 6 contained 3.7 mg.l "1 NEj-N, and 8.7 mg.l "1 12 hours after Example 7, indicating that the system had not completely recovered, but this was partly attributed to ambient temperatures of 5 - 8 °C. Example 8 - Nitrification inhibition by toxic substance Example 8 was a chemical shock applied to one of the activated sludge systems. A dose of allylthiourea (ATU) , a known nitrification inhibitor was mixed with sewage and pumped in as feed over a 15 minute period. The total ATU load was 22.8 mg, a dose calculated to give 75% inhibition in the aeration basin. A concentration of 0.076 mg.l "1 produces 75% inhibition. Initially the influent NH 3 -N concentration was 38.1 mg.l "1 and MLDO was 2.2 mg.l "1 . The aeration rate was kept constant throughout the 12 hour test, and MLDO, headspace N 2 0 and effluent N0 3 -N, N0 2 -N and NH 3 -N were measured as in Examples 5 to 7. In Example 8, the MLDO and N 2 0 increased when the ATU dose was applied to the aeration tank. Maximum mixed liquor dissolved oxygen (MLDO) was reached approximately 180 minutes after the shock load, N 2 0 after approximately 135 minutes. Both DO and N 2 0 recovered after approximately 10 hours. Effluent N0 2 -N (Figure 10a) and NH 3 -N (Figure 10b) remained stable at 3.8 ± 0.1 mg.l "1 and 2.5 ± 0.3 mg.l "1 respectively until 7 hours after the shock load was applied. The increase in exhaust gas N 2 0 preceded the appearance of ammonia and nitrite in the plant effluent by approximately 6 hours (i.e. 0.75 x HRT) . Toxicity can be caused by other substances including heavy metals, organic compounds with C-S and C-N linkages, phenolics and sulphides. The results are summarised in the table below. The time lag between N 2 0 detection and the appearance of nitrite in the effluent ranged from 0.75 to 0.90 HRT. Mean time taken from N 2 0 detection to appearance of nitrite was 0.808 HRT (i.e. 6 hours 28 minutes) , providing early warning of nitrification failure. Resul ta summary- • Example Type Initial Peak Time to Time to Time to rime from w 2 o N 2 0 N 2 0 effluent increased N 2 0 (ppm) (ppm) peak N 2 0 effluent increase (HRT) (HRT) NH3 to (HRT) effluent N0 2 (HRT) 4 NH 3 0.1 17.5 0.0625 0.9 0.9 0.9 5 NH 3 0.1 14.2 0.0313 0.938 0.938 0.78 6 MLDO 0.1 22.3 0.125 0.938 0.938 0.805 7 MLDO 0.1 28.1 0.135 0.938 0.938 0.805 8 ATU 0.1 54.5 0.155 0.875 0.875 0.75 From all of the above Examples it can be seen that N 2 0 detection provides early warning of nitrification failure, regardless of the cause of the failure. The time lag between detection of increased offgas N 2 0 and the appearance of nitrite in the effluent ranged from 0.75 to 0.90 HRT. The mean time taken from N 2 0 increase to appearance of nitrite was 0.808 HRT, providing a 6.5 hour warning of nitrification failure, indicating that N 2 0 detection is a superior indicator of nitrification failure.

Claims: CLAIMS 1. A method comprising nitrification and measuring N 2 0 produced. 2. A method according to claim 1, wherein said nitrification is nitrification of waste water. 3. A method according to claim 1 or 2 , wherein said nitrification comprises the addition of 0 2 and nitrifying bacteria to said waste water. 4. A method according to any preceding claim, wherein said nitrification is failing, the failure is indicated by increase in the N 2 0 measured, and action is taken in response to that increase. 5. A method according to claim 4, wherein said failure is produced by a toxic substance . 6. A method according to claim 4 or 5 as appended to claim 3, wherein said action comprises increasing the rate of 0 2 addition. 7. A method according to claim 4 or 5, wherein said action comprises diverting said waste water to a storage tank. 8. A method according to claim 7, and further comprising treating the diverted waste water. 9. A method according to claim 4 or 5, wherein said action comprises reducing the inflow of waste water to the nitrification.

Inventors: STEPHENSON THOMAS, STEPHENSON, THOMAS
Applicants: STEPHENSON THOMAS, UNIV CRANFIELD, CRANFIELD UNIVERSITY, STEPHENSON, THOMAS
Application year: 2000
Main classification: C02F3
IPC codes: C02F3