NZ535179A - Nitrate removal in waste water systems - Google Patents

Abstract

A method of controlling a denitrification process is disclosed, which comprises the steps of: monitoring the nitrate concentration of waste water entering a containment area, the containment area containing a carbon source such as woodchips, sawdust, activated carbon, or other vegetable based materials, monitoring the nitrate concentration of waste water exiting the containment area, monitoring the temperature within the containment area, and adjusting the flow rate of waste water entering the containment area based on the nitrate concentration of waste water entering and exiting the containment area and the temperature within the containment area so as to achieve a required nitrate removal efficiency. Also disclosed is a denitrification system comprising: a containment area with an impermeable boundary on at least the bottom and sides of the containment area, the containment area at least partially filled with a carbon source, the containment area arranged to remove nitrates from waste water containing nitrates, an inflow pipe to allow waste water containing nitrates to flow into the containment area, an outflow pipe from the containment area, the inflow pipe and outflow pipe arranged to provide gravity drainage to the containment area.

Description

53517 NEW ZEALAND PATENTS ACT, 1953 No: 535179 Date: 8 September 2004 COMPLETE SPECIFICATION NITRATE REMOVAL IN WASTE WATER SYSTEMS We, INSTITUTE OF GEOLOGICAL & NUCLEAR SCIENCES LTD, a New Zealand company of 30 Gracefield Road, Lower Hutt, New Zealand, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: INTELLECTUAL PHOPWTY OFFICE OF NX -1 - - 8 SEP 2005 RECEIVED NITRATE REMOVAL IN WASTE WATER SYSTEMS FIELD OF INVENTION The invention relates to the denitrification of waste water and in particular to the denitrification of municipal effluent using carbon based material.

BACKGROUND One of the components of waste water is nitrogen. Nitrates leaching into waterways can lead to the growth of undesirable algae and other aquatic plant life. Alternatively nitrogen leached as ammonia can be toxic to aquatic life. For environmental reasons the nitrogen content of waste water must be reduced as far as possible.

One example of an existing system for reducing nitrogen levels in waste water is a recirculating sand filter with anaerobic filter and carbon source. In this system waste water is first passed through a septic tank and then into a recirculation tank. From the recirculation tank the waste water passes through a sand filter. After passing through the sand filter some waste water is fed back to the recirculation tank. Carbon from a carbon source is added to the remaining waste water which then passes into an anaerobic filter. The waste water from the anaerobic filter is then passed to subsurface infiltration and optionally to surface discharge. These systems normally remove 40 to 50% of influent nitrogen.

US patent 5,288,407 describes another denitrification system. The process of US 5,288,407 involves waster water receiving an initial treatment in a settling basin (which may be a septic tank). Effluent waste water from the settling basin flows into an aerobic, free-draining nitrification field either by fixed-bed gravity or by pumping. The nitrification field consists of a graded media through which waste water enters at the top. After passing through the graded media and becoming nitrified, waste water is collected at the bottom of the nitrification field. This waste water flows by gravity to a flow splitting device that divides the waste water flow two streams. One waste water 204672_2.DOC 2 flow stream is recirculated back to the nitrification field by pumping; the other waste water flow stream flows by gravity to the denitrification chamber which utilizes sulphur and limestone as denitrification media. After passing through the denitrification chamber, denitrified waste water is conveyed to a discharge system. This denitrification 5 system relies on endogenous carbon for the denitrification process.

SUMMARY OF INVENTION It is the object of the invention to provide an improved or alternative denitrification 10 process or to at least provide a useful choice of denitrification processes.

In broad terms in one aspect the invention comprises a method of controlling a denitrification process comprising the steps of monitoring the nitrate concentration of waste water entering a containment area, the containment area containing a carbon 15 source, monitoring the nitrate concentration of waste water exiting the containment area, monitoring the temperature within the containment area, and adjusting the flow rate of waste water entering the containment area based on the nitrate concentration of waste water entering and exiting the containment area and the temperature within the containment area so as to achieve a required nitrate removal efficiency.

Preferably the method of controlling the denitrification process further comprises using a computer to determine the nitrate removal efficiency.

Preferably the computer controls a valve to adjust the flow rate of waste water entering 25 the containment area.

In broad terms in a further aspect the invention comprises a denitrification apparatus comprising a containment area with an impermeable boundary on at least the bottom and sides of the containment area, the containment area at least partially filled with a 30 carbon source, the containment area arranged to remove nitrates from waste water containing nitrates, an inflow pipe to allow waste water containing nitrates to flow into the containment area, an outflow pipe from the containment area, the inflow pipe and outflow pipe arranged to provide gravity drainage to the containment area. 204672 2.DOC intellectual property office of n.z. \ 2 OCT 2005 received Preferably the containment area is below ground level.

Preferably the containment area is covered with a layer of geo-fabric.

Preferably the dimensions of the containment area are determined by the waste water volume, the waste water temperature, the atmospheric temperature and the sustainable nitrate concentration in the waste water entering the containment area.

Preferably the containment area is an elongated trench.

Preferably the carbon source in the containment area comprises one or more of woodchips, sawdust, activated carbon, or other vegetable based materials.

BRIEF DESCRIPTION OF DRAWINGS The invention will be further described by way of example only and without intending to be limiting with reference to the following drawings, wherein: Figure 1 is an overview of a denitrification trench and plumbing design; and Figure 2 is an overview of a real-time nitrate monitoring system.

DETAILED DESCRIPTION Figure 1 shows an overview of a denitrification trench and plumbing design. Waste water enters the denitrification containment area 1 through inflow line 2. After processing in the trench the waste water exits the containment area 1 through outflow line 3.

In the embodiments shown in Figures 1 and 2 inflow line 2 and outflow line 3 are pipes. In the embodiment of denitrification trench shown in Figure 1 the waste water flows 204672_2.DOC 4 into and out of the containment area via a gravity feed system so that waste water flows down into the containment area 1 and flow down out of the containment area 1. An advantage of using a gravity feed system for the containment area is that it is cost efficient to construct as no pumps are required to remove waste water from the 5 containment area. Pumping may be required into the bed depending on the height difference between the upstream treatment plant (not shown) and the bed and the waste water flow rate required to be input through the bed. In alternative embodiments other feed systems may be used to supply waste water to the containment area.

Containment area 1 includes an impermeable membrane liner 4 that prevents direct seepage of waste water from the sides and bottom of the containment area into the soil and groundwater system surrounding the containment area. The containment area is filled with carbon source 5. The carbon source may be any suitable material but in preferred embodiments is one or more of; wood chips, sawdust, activated carbon, or any 15 other suitable vegetative based material. In one embodiment the carbon source is Radiata pine wood chips and sawdust. In the preferred embodiment the containment area is non-baffled. In this embodiment the residence time of the waste water within the containment area is determined by the hydraulic conductivity of the carbon source, the containment area dimensions and the waste water inflow rates. The carbon source must 20 have appropriate hydraulic conductivity to maintain trench drainage and waste water residence time in the containment area. In alternative embodiments the waste water residence time may be controlled by other means.

Nitrate is removed from the waste water by anaerobic biological activity. The anaerobic 25 biological activity is provided by natural microbes that become very active in environments rich in carbon, from the carbon source, and nitrate, from the waste water. The anaerobic activity produces the following denitrification reaction: 5CH20 + 4 NO" -> 5C02 + 2N2 + 3H20 + 40H" The above reaction results in nitrate (NOj) being converted into N2 gas. The optimisation of nitrogen removal in the containment area requires maximising the nitrate component of nitrogen in the waste water at the treatment plant. 204672_2.DOC The optimum volume of the containment area and the carbon source within the containment area is dependent on several factors including the expected maximum waste water volume per day, the maximum instantaneous waste water flow rate and the long term sustainable waste water nitrate concentration discharged from the treatment 5 plant. As a rule of thumb as the expected maximum waste water volume per day increases so will the required containment area and carbon source volumes. As the maximum instantaneous waste water flow rate increases so will the required containment area and carbon source volumes. As the sustainable waste water nitrate concentration discharged from the treatment plant decreases the required containment 10 area and carbon source volumes will increase.

In preferred embodiments, as shown in Figure 1, a geo-fabric 6 covers the carbon source 5 within the containment area 1. The geo-fabric prevents potential erosion of the carbon source. The geo-fabric 6 may be covered with a layer of material, for example bark 7, 15 to provide a visual aesthetic cover to the containment area. In alternative embodiments the geo-fabric and covering material may be omitted. The geo-fabric is permeable to allow rainfall to infiltrate the containment area and nitrogen gas to escape the containment area. Typically the geo-fabric is synthetic.

The reaction rate for converting nitrate into nitrogen gas in the containment area is dependent on temperature and availability of carbon. The reaction rate decreases with decreasing temperature and/or a reduction in carbon availability. Temperature probe 8 is provided to monitor the temperature within the containment area. In alternative embodiments more than one temperature probe may be provided within the containment 25 area. It is preferred that if more than one temperature probe is provided within the containment area the temperature probes are substantially evenly spaced within the containment area to provide the most accurate possible indication of temperature within this area. In the preferred embodiment the temperature probe is installed in a piezometer.

The concentration of nitrate in the waste water entering the containment area 1 is monitored by upstream monitoring chamber 9 on the inflow line 2. The concentration 204672J.DOC 6 of nitrate in the waste water exiting containment area 1 is monitored by downstream monitoring chamber 10 on the outflow line 3. A flow valve 11 is also provided on the inflow line 2 to control the flow of waste water into the monitoring chamber 9 and containment area 1.

Figure 2 shows a real time monitoring system for the denitrification treatment system of the invention. The monitoring system includes a computing device 17 that receives information from nitrate probes 13, 14 on the inflow and outflow lines 2 and 3 respectively and from temperature probe 8.

Computing device 17 may be positioned close to the denitrification treatment system or may be remote from the denitrification treatment system. The computing device may be a computer, microprocessor, microcontroller or any suitable device. If the computing device is close to the denitrification system connections 18 and 19 between the nitrate 15 probes, temperature probe, flow meter and the computing device may be wired. Alternatively connections 18 and 19 may be wireless. It should be noted that while connection 18 between the nitrate probes, temperature sensor and the computing device is shown as a single connection this represents a connection between each nitrate probe and the computing device and a connection between the temperature sensor and the 20 computing device.

The upstream monitoring chamber 9 and the downstream monitoring chamber 10 house not only nitrate probes 13, 14 but also electronics to communicate readings from the nitrate probes to computing device 17. If the connections between the monitoring 25 chambers and the computing device are wireless the electronics in the monitoring chambers may include cellular telephones or radio telemetry. Electronics in the monitoring chambers may be arranged to respond to a query from the computing device or to send information representing probe readings to the computing device at preset time intervals (for example at one minute intervals). Alternatively the electronics could 30 be arranged to send information representing probe reading to the computing device if the probe reading are outside preset limits. In yet other alternative embodiments a 204672 2.DOC 1 combination of these arrangements or any other suitable probe information reporting arrangement may be used.

The temperature probe(s) 8 is/are also attached to electronics to communicate readings 5 from the temperature probe(s) 8 to the computing device. The electronics may include cellular telephones or radio telemetry if the computing device is wirelessly connected to the electronics. Electronics attached to the temperature probe(s) may be arranged to respond to a query from the computing device or to send information representing probe readings to the computing device at preset time intervals. Alternatively the electronics 10 could be arranged to send information representing probe reading to the computing device if the probe reading are outside preset limits. In yet other alternative embodiments a combination of these arrangements or any other suitable probe information reporting arrangement may be used.

Flow valve 11 is also attached to electronics that control the flow valve. The electronics may include cellular telephones or radio telemetry if the computing device is wirelessly connected to the electronics. The electronics are arranged to respond to a signal from computing device 17 via connection 19 and control the flow valve to control the flow rate of waste water into the containment area.

In use computing device 17 receives data from nitrate probes 13 and 14 and temperature probe(s) 8 corresponding to the nitrate rate of waste water flowing into the containment area, the nitrate rate of waste water flowing out of the containment area and the temperature within the containment area respectively. The computing device then 25 calculates the denitrification efficiency of the containment area for the current waste water flow rate, the waste water nitrate concentration and the containment area temperature. Nitrate removal efficiency is determined as the difference between the inflow and outflow nitrate concentrations as measured by nitrate probes 13 and 14. If the nitrate removal efficiency of the containment area is less than a predefined 30 percentage (for example 99%) the computing device sends a signal to the flow meter to reduce the flow rate of waste water into (and out of) the containment area. The flow rate is reduced to an appropriate rate to maintain the nitrate removal efficiency of the 204672_2.DOC 8 containment area above the predefined percentage. The new flow rate is based on a calculation that takes into account the containment area temperature and the waste water residence time inside the containment area. The denitrification system efficiency to reduce total nitrogen concentration of the waste water requires pre-trench treatment 5 maximising the conversion of nitrogen species into the oxidised form of nitrate before entering the containment area.

For example the temperature in the containment area may fluctuate. As the containment area temperature fluctuates the reaction rate in the containment area also fluctuates 10 leading to a fluctuation in the denitrification system efficiency. This fluctuation in temperature and efficiency is detected by the computing system which alters the flow rate into and out of the containment area to maintain minimum nitrate removal efficiency.

Over time, with prolonged usage the availability of carbon in the containment area may decrease. This decrease in carbon availability leads to a decrease in denitrification efficiency that is independent of efficiency changes due to temperature fluctuations. The change in denitrification efficiency is detected by the computing system which adjusts the flow rates into and out of the containment area accordingly. The computing 20 system may also provide an estimate of the carbon availability in the containment area and provide an alert when the carbon availability needs to be increased.

The containment area and monitoring system provides for optimisation of the containment area size based on the estimated peak waste water loading rates. The 25 system maximises nitrate removal from the waste water by adjusting the inflow rates to maintain denitrification efficiency.

The term 'comprising' as used in this specification and claims means 'consisting at least in part of, that is to say when interpreting statements in this specification and claims 30 which include that term, the features, prefaced by that term in each statement, all need to be present but other features can also be present. 204672_2.DOC 9 The foregoing describes the invention including preferred forms thereof. Alterations and modifications as will be obvious to those skilled in the art are intended to be incorporated in the scope hereof. 204672J.DOC

Claims (13)

1. A method of controlling a denitrification process comprising the steps of: monitoring the nitrate concentration of waste water entering a containment area, 5 the containment area containing a carbon source, monitoring the nitrate concentration of waste water exiting the containment area, monitoring the temperature within the containment area, and adjusting the flow rate of waste water entering the containment area based on the nitrate concentration of waste water entering and exiting the containment area and the 10 temperature within the containment area so as to achieve a required nitrate removal efficiency.

2. A method of controlling a denitrification process as claimed in claim 1 further comprising the step of using a computer to determine the nitrate removal efficiency. 15

3. A method of controlling a denitrification process as claimed in claim 2 wherein the computer controls a valve to adjust the flow rate of waste water entering the containment area. 20

4. A denitrification system comprising: a containment area with an impermeable boundary on at least the bottom and sides of the containment area, the containment area at least partially filled with a carbon source, the containment area arranged to remove nitrates from waste water containing 25 nitrates, an inflow pipe to allow waste water containing nitrates to flow into the containment area, an outflow pipe from the containment area, the inflow pipe and outflow pipe arranged to provide gravity drainage to the 30 containment area.

5. A denitrification system as claimed in claim 4 wherein the containment area is below ground level. 2Q4672_2.DOC \ \ intellectual property office of n.z. 1 2 OCT 2005 received 10

6. A denitrification system as claimed in claim 4 or claim 5 wherein the containment area is covered with a layer of geo-fabric.

7. A denitrification system as claimed in any one of claims 4 to 6 wherein the dimensions of the containment area are determined by the waste water volume, the waste water temperature, the atmospheric temperature and the sustainable nitrate concentration in the waste water entering the containment area.

8. A denitrification system as claimed in any one of claims 4 to 7 wherein the containment area is an elongated trench.

9. A denitrification system as claimed in any one of claims 4 to 8 wherein the carbon source in the containment area comprises one or more of woodchips, sawdust, activated carbon, or other vegetable based materials.

10. A denitrification system as claimed in any one of claims 4 to 9 further comprising a computing system arranged to determine nitrate removal efficiency.

11. A denitrification system as claimed in claim 10 wherein the computing system is arranged to control the flow rate into the containment area.

12. A method of controlling a denitrification process, substantially as herein described with reference to the accompanying drawings.

13. A denitrification system, substantially as herein described with reference to the accompanying drawings. —Q\- ^ K.\. \ / i ^ ~ ^ ^ By the authorised agents A. J. PARK 204672 2.DOC 12