Landfill Leachate Treatment Manual

 

1.         Introduction

The treatment of wastewater including landfill leachare is accomplished by developing an aerobic bacterial biomass in the aeration lagoons and insuring that the bacteria actively metabolise the dissolved components comprising of organic matter and nitrogenous matter present in the leachate or wastewater.

The organic metabolising bacteria are heterotrophic, and the inorganic nitrogen metabolising bacteria are autotrophic.  Both groups of bacteria are aerobic and require a supply of oxygen, which is provided by the fine bubble diffused aeration system provided by Dryden Aqua.  The aeration system and nutrient dosing is a life support system designed to maintain the bacteria that perform the task of treating the wastewater.

1.1      Key points

  • bacterium nutrient requirements
  • oxygen
  • environmental conditions
  • trace nutrients
  • time & temperature

1.2     Bacterium nutrient and oxygen

The aerobic bacteria use the waste matter in the water as a food source.  The heterotrophic bacteria feed on organic matter to reduce the COD (Chemical Oxidation Demand) of the water.  As a guide 1 kg of COD reduction will require 2 kg of oxygen. 10Kg of COD will also remove 1 kg of ammonium nitrogen.  Heterotrophic bacteria develop very rapidly at a water temperature of 20 deg C, the bacteria can double every 30 minutes.  The bacteria are also active at low water temperature and will keep working down to 5 deg C. The addition of an organic source such as molasses is therefore a useful strategy to employ in order to reduce ammonium levels at low temperatures.

Autotrophic nitrifying bacteria use inorganic carbon in the form of carbonates as a carbon source, and ammonium as an energy source.  The bacteria convert ammonium to nitrate. Every 1 Kg of ammonium metabolised requires 7 kg of carbonate as CaCO3, and 5kg of oxygen. The air diffusers provide the oxygen and some of the inorganic carbon in the form of carbon dioxide from the atmosphere.  However there is often a short fall in supply of inorganic carbon, and as a consequence the pH of the leachate becomes acidic. Under acidic conditions, there is no inorganic carbon in the water and nitrification of ammonium will slow down and stop.  It is therefore very important to always maintain the pH of the water above pH 7.

Nitrifying bacteria are very effective in removing ammonium, however at water temperature below 8 deg C, bacterial activity is greatly reduced and more reliance may need to be placed on heterotrophic bacteria for ammonium nitrogen control.


Table 1.2.1. Relationship between Ammonium, BOD, Phosphate and alkalinity

KGs of component required Ammonium nitrogen 1kg COD/BOD 1 kg Phosphate 1kg
Oxygen required in Kg 5 Kg 2 Kg ***
Ammonium nitrogen consumed Kg *** 0.1 Kg 10 Kg
COD/BOD consumed Kg 10 Kg *** 100 Kg
Phosphate required Kg 0.1 Kg 0.01 Kg ***
Alkalinity required The alkalinity of Polmaise leachate water is high, concentration 5000 mg/l, or 5 kg for cubic meter as calcium carbonate.  There is therefore a short fall of 2 kg of CaC03. The air diffusers dissolving carbon dioxide into the water will make up a high percentage of the short fall.  The remainder needs to be satisfied by the addition of extra alkalinity in one of the forms given opposite. 
Sodium hydroxide, or 4 kg (12 litres)
Sodium bicarbonate, or 12 kg
Calcium carbonate, or 7 kg
Magnesium oxide 3 kg

Alkalinity buffering equation

1. H20 + CO2      <=>     H2CO3  <=>        HCO3 + H+   <=>       CO3 + 2H+

Nitrification equations

2. NH4+ + 1.5O2     =>     2H+ + 2H2O + NO2-

3. NO2- + 0.5O2    =>      NO3-

4. NH4+ + 1.83 O2 + 1.98 HCO3-   =>       0.021 C5H702N + 0.98 NO3- + 1.041 H2O + 1.88 H2CO3-

5. NH4+ + 1.9O2 + 2HCO3-    =>      1.9 CO2 + 2.9 H2O + 0.1 CH2

from 1 gram of ammonium.

8.59 grams of carbonic acid is produced (H2CO3)

0.17 grams of autotrophic nitrifying bacteria cells are produced. 

Fig 1.2.1 Relationship between Carbonate (CO3) Bicarbonate (HCO3) and Carbon dioxide (CO2)

 

 

1.3      Environmental conditions

The leachate provides the food for the bacteria and the aeration system provides the oxygen.  However, bacteria like to form colonies, and these colonies of bacterial floc need to be kept in suspension.  If the floc are allowed to settle out in the aeration vessel, the sludge would become anaerobic and the aerobic bacteria would be destroyed.

The shape and structure of the bacterial floc are also important.  The larger the floc, and the greater the species diversity of bacteria and the better the system performs.  For intensive aeration systems with a resodence time under 5 days, we recommend the application of a fluidised bed biofiltration media.  The system is then operated as a fluidised be biofilter.

Fine bubble air diffusers do not waste kinetic energy in directly moving water or throwing water into the air.  Also, as bubble size is reduced, the bubble surface area increases exponentially which increases the diffusion of oxygen into the water.  The smaller the bubble, the greater the friction between the rising bubble and the water and more water is circulated per unit volume of air diffused.  It is important that the water in the aeration vessel is kept moving at a rate that maintains the bacterial floc in suspension.  However the rate of water movement and the means of achieving the task should not break-up the bacterial floc.

The air diffusers will be subject to some degree of bacterial and chemical fouling. In water over 400mg/l as CaCO3, some of the carbonate will precipitate out of solution onto the surface of the diffusers.  In hard leachate water it will be required to remove the diffusers and soak in a dilute solution (5% total solution) of phosphoric acid for approximately 15 minutes. Our air diffusers are used in waste water with an alkalinity level of over 5000mg/l, carbonate precipitation will occur on the diffusers in Lagoon 1. By the time the waste water reaches the second lagoon, the alkalinity level will be much lower, but there will still be some degree of diffusers bacteria fouling, it is therefore useful to remove the diffusers once a month for a quick inspection and brush to remove the deposits.  The process just takes a few minutes,  pull the diffuser out of the the tank using the 1/2" supply hose,  give it a quick brush and shake, and throw back into the tank or lagoon

Bacterial biomass

It is important that the bacteria are not lost from the system. Simple vertical tube clarfiers are used to keep the bacteria in the aeration vessel. The vertical clarifier pipe diameter is sized such that the vertical velocity of the leachate being discharge from the tank is slower than the rate at which the bacteria floc settles.   Granular activated carbon may be used to improve floc structure in stressed systems.  If the residence time is over 5 days, the bacterium should be in endogenous respiration and it is unlikely there will be any sludge development.

1.4      Trace nutrients

The wastewater may not provide all of the nutrients required by the aerobic bacteria, these nutrients therefore have to be added to the aeration vessel.  The principle nutrient is usually phosphate that can be added as phosphoric acid.

1.5      Time and temperature

The ability of the bacteria to treat the wastewater is a function of the bacterial activity and time the bacteria have to achieve the task.

Biological activity is very closely related to water temperature. From 10 to 30 deg C biological activity can double for every 5 deg C increase in temperature.  The autotrophic nitrifying bacteria tend to be much more sensitive than the heterotrophic bacteria. Below 8 deg C and above 35 deg C nitrification is inhibited.   The heterotrophic bacteria are much more temperature tolerant and will keep working down to 5 deg C.


2          System Design Concepts

2.1      Out-line design.

The site limitation and infrastructure available on the site have dictated the design and size of  the treatment system.

2.1.2    Lagoon shape

Lagoons are a very cost effective means of providing large aeration vessels.  The optimum water depth is 3 to 5metres.  

2.1.3    Lagoon size and number

Heterotrophic bacteria can metabolise organic matter very rapidly. 80% reduction could be achieved in less than 24 hours.  However in leachate there will be organic mater that is hard for the bacteria to break down.  This matter is referred to as hard COD.  However given a stable healthy bacterial cell biomass with a high species diversity, organisms will develop in the bacterial floc that will become adapted to dealing with hard COD.   There will be a low biomass of the more exotic bacteria, and they will take longer to digest the hard COD, so the longer the time given for these organisms to complete the task, the more effective the treatment system. 

Autotrophic nitrifying bacteria are extremely slow growing.  At a water temperature of 15 deg C the doubling time for the bacteria will be in the region of 30 days, at 30 deg C it will be 2 days.  It is therefore important that the bacteria form a stable bacterial floc in cold water systems, and that they are prevented from escaping from the system.  Autotrophic bacteria can also be shock loaded and inhibited if the ammonium levels are increased above 80 mg/l as NH4-N.   The wastewater should therefore be added at a slow constant rate to the lagoons or aeration tanks.  The larger the lagoons the more stable the environmental conditions and the lower the ammonium levels.  It is also useful to interlock a dissolved oxygen probe to the supply of waste water. Waste water addition is reduced and stopped ifd the dissolved oxygen concentration falls rto 2mg/l.  It is essential that the aeration lagoon does not drop below 2 mg/l.

Nitrifying bacteria can become overwhelmed by the faster growing heterotrophic bacteria. The population density of heterotrophic bacteria is related to the amount of organic matter present in the leachate.  The lagoons are connected in series, lagoon 1 will therefore see a higher concentration of organic matter and population of heterotrophic bacteria. The heterotrophic bacteria in lagoon 1 will assimilate ammonia, there will also be some nitrification in the same lagoon.  However by connecting the lagoons in series, the water quality environmental conditions will be different between lagoon 1 and lagoon 2. The different conditions promote a higher species diversity of organisms, which in turn promotes a higher capacity and performance of the system.

2.2      Size of aeration system

Using the dryden Aqua air dffusers  provides a very high degree of flexibility.  A system from 1 to over 1000 diffusers is easily installed.  As a guide for a municipial waste water treatment system 1 x diffuserr supplies sufficient oxygen for 50 to 100PE (People Equivalent). 1 x diffuser and 10 m3/hr of air delivers between 1 and 2 kg of oxygen per hour.

2.3      Treatment capacity

The treatment capacity of the system is related to the amount of oxygen available for the aerobic bacteria.

The amount of oxygen that dissolves into the water is a function of the transfer efficiency of oxygen from the air to the water.  The efficiency is related to a range of factors including, the ionic and surfactant content of the water, temperature, depth of diffusers and the oxygen tension in the water.

The transfer efficiency is very closely linked to the partial pressure or oxygen tension in solution. For example as the oxygen concentration approaches 100% saturation, the transfer efficiency of oxygen going into solution approaches zero.  Conversely as the oxygen partial pressure approaches zero, the transfer efficiency increases.  The net result is that the partial pressure of oxygen in solution will find an equilibrium value in which the consumption of oxygen by the bacteria equates with the transfer of oxygen from the diffusers. 

At start up of the system there will be a low bacterial cell biomass in the lagoons, and as such the equilibrium dissolved oxygen pressure will be close to 100% saturation. However as the bacterial cell biomass increases the equilibrium oxygen level will fall, and the transfer efficiency of the diffusers will increase exponentially.

3. Operating, Maintenance & Chemical dosing schedule

The following is an example of a system, retrofitted with Dryde Aqua air diffusers.  The existing syste was in effective using surface aerators, the aeratoirs were replaced with Dryden Aqua air diffusers and for the same energy expenditure,  the transfer of of oxygen increase by 100%.

 

Fig.3.1 Hydraulic flow diagram of treatment system


 

 

3.1 Hydraulic flow diagram of a typical wastewater or  leachate treatment system.

Lagoon 1

The raw leachate from the site enters lagoon 1 via a slam shut valve. The valve is controlled by water level and the dissolved oxygen content of lagoon 1.  The valve will open when the leachate level in lagoon 1 drops, and conversely as the leachate level rises the valve adding raw leachate to lagoon 1 will close.

Lagoon 1 overflows into lagoon 2 via a vertical tube clarifier, the clarifier minimises the transfer of bacteria floc from lagoon 1 to lagoon 2.

The raw leachate contains ammonium and organic matter which acts as a food source for the bacteria.  In order to digest the food, the bacteria need oxygen. An oxygen probe in lagoon 1 monitors and controls the dissolved oxygen level in the leachate.  If the oxygen levels fall below 3mg/l, the oxygen meter will close the slam shut valve.  It is therefore impossible to overload the system with leachate.  It is important to make sure the probe is kept clean and to confirm that it is giving the correct reading.

Lagoon 2

Lagoon 2 receives leachate from lagoon 1, as leachate is added to lagoon 1 it simply displaces the same volume of leachate into lagoon 2.  Lagoon 2 is fitted with three dosing pumps, two small pumps for dosing antifoam, and phosphoric acid.  The third larger pump may be used to dose caustic. Fig 3.1 also shows a molasses pump, this pump is not fitted, but it may be considered at a latter date once the molasses dosing requirement of the system has been established.

Lagoon 2 is fitted with a submersible pump, the pump delivers leachate from lagoon 2 to the discharge tank. A high level float switch in the discharge tank is interlocked to the pump.  The float switch will turn the pump off when the water level reaches the upper level float.

Discharge Tank

The discharge tank receives leachate from lagoon 2 when the water level in the discharge tank is below the upper level float switch.  The discharge tank is operated as a sequencing batch reactor SBR.  One or more times each day the PLC controller will turn off the air to the discharge tank.  The solids in the tank are allowed to settle for a period of 1 hour, after which the discharge valve will open to allow the supernatant treated leachate in the discharge tank to be transferred to the discharge settlement channel. After a period of 2 hours, or if the water level drops to the low level float switch, the discharge valve will close and the air valve will open.

The quantity of leachate discharged depends upon the distance between the upper and lower level float switches in the discharge tank, and the aeration time set on the PLC controller. The manual for the PLC controller has been supplied as a separate document.

3.2 SBR operating parameters

Lower float switch in tank to be 1m below the upper float switch to give a discharge treated leachate volume of approximately 50 cubic metres of effluent per cycle.  The position of the float switch may be adjusted to alter the quantity of leachate discharged. Raising the lower float switch will reduce the quantity of leachate, and lowering the low level float switch will increase the quantity of leachate discharged per cycle.  Do not adjust the upper level float switch.

3.3 SBR control the system

The quantity of leachate that can be treated is related to the performance of the system in achieving compliance with the discharge consents.  The principle parameters are the suspended solids and ammonium concentration.

Stop all discharge.

In order to prevent the discharge of leachate, the manual 200mm butterfly valve on the discharge pipe is closed.  Normally this valve will be cracked open to allow the discharge of approximately 50 to 100 cubm of treated leachate per hour.

 

 

Controlled discharge

The standard settings for the controller are as follows;

Aeration phase                        9 hours

Settlement phase                     2 hours

Discharge phase                      1 hour

 

 

Each cycle will discharge 50 cubm of effluent, based on a 9 hour aeration phase there will be two cycles over a 24 hour period and a total discharge of approximately 100 cubm/day.

The principal controlling factor will likely be the concentration of ammonium in the effluent.  The ammonium levels should be measure each day in Lagoon 1, Lagoon 2 and Discharge tank.  The information should be recorded and stored on a spreadsheet.

The objective is to try and achieve a steady state situation with regards to the concentration of ammonium in each of the lagoons and discharge tank.  If the conditions are kept constant the bacteria will adapt to the conditions and the performance of the system will improve.  In order to try and achieve steady state conditions, the aeration time can be altered.

 

Table 3.1 Aeration time verses daily discharge, with low level float switch 1m below upper level float switch in discharge tank

Aeration interval in hours Average cubm/day discharged
45 25
21 50
16 75
9 100
5 150
3 200

 

In order to increase the discharge water flow above 200 cum/day it will be necessary to drop the low level float switch in order to increase the amount of leachate discharged per cycle.  The effluent discharge ditch should be able to hydraulically cope with 100 cubic metres of effluent over a 1 hour period.

 

Table 3.2 Aeration time verses daily discharge, with low level float switch 2m below upper level float switch in discharge tank

Aeration interval in hours Average cubm/day discharged
21 100
16 150
9 200
5 300
3 400

 

3.4 Ammonium levels and discharge volume

According to Table 3.1, an aeration interval of 9 hours gives a discharge volume of 100 cubm/day. However if the ammonium levels are well under compliance, the time interval may be reduced to 8 hours, for several days, and then down to 7,6 or 5 hours if the effluent water quality is sustained. Alternatively the low level float switch can be dropped in small increments to increase the amount of leachate discharge while keeping the cycles at the same time interval.   It is important to make small steps at a time to keep the ammonium levels constant. 

If the ammonium levels are rising then the aeration time interval will need to be increased.  The aeration time interval can be increased quicker than it can be decreased.

Example.

Aeration interval 9 hours

Ammonium levels are increasing rapidly in the system due to a change in the raw leachate quality with ammonium increasing from 600 to 2000 mg/l.  Ammonium levels in discharge tank are only 5mg/l however levels in Lagoon 2 have increased to 120mg/l.

Use the ammonium levels in lagoon 2 as an early warning indicator and adjust the aeration time to slow down the rate of discharge until lagoon 2 drops below 75mg/l. 

Reduce aeration interval to 16 hours, this will reduce the amount of leachate entering the system and will allow the levels to drop in lagoon 2. If after several days there is no change or if the levels are rising rapidly in the discharge tank, increase aeration interval to 21 hours.  If the ammonium levels continue to rise and there is risk of failing consent condition, close manual valve or increase aeration interval to the maximum of 99 hours.

Once the ammonium levels have stabilised, and only once lagoon 2 is below 80mg/l and the discharge tank is well in compliance should the aeration time interval be reduced to permit more discharge from the tank.


3.5 Checks & Maintenance schedules

Table 3.5.1 maintenance and checking schedule

  daily weekly monthly
Lagoon water levels The water level in lagoon 1 & 2 should be the same. The high level float base should be just in water and at an angle of about 45 deg.    
Lagoon 1 Oxygen probe   See appendix 3  

Lagoon 1

High level float switch lagoon 1

  Remove float switch from lagoon and tilt. You should hear the slam, shut valve activate.  If the valve does not work, check for air pressure at the valve, or float switch wiring. Replace float switch after 36 months

Lagoon 2

Submersible pump

  Check that the pump is functioning properly.  If the pump is not working, there will be no discharge of leachate from the site, so it should be obvious.  

Lagoon 2

Dosing pumps

Check that the dosing pumps are primed and working. There can be problems keeping the pumps primed during the winter months because the chemicals become more viscous  

Discharge tank

High level float

  The system will discharge the same volume of leachate each day. Take a note of the leachate volume. If quantity is too much or little, check the position and operation of the float switches  located on top of the discharge tank.  Adjust the height of the low level float to control the amount of leachate discharged. Lowering the float 100mm increases discharge by 5 cubic metres day. Replace float switches after 36 months

Discharge tank

Low level float

 
Air Blower gauges and bearings Check to insure the air pressure is below 800mbar. If air release is venting, clean the diffusers. Also check for water in the air ring main. Check the air intake filters, clean or replace as necessary Change air filter every 3 to 6 months.
Air blower oil and belts Read the manufacturers manual for full details. When blower is off, Check that no belts have broken. Also make sure that the oil level is okay. It should be half way up the sight glass. If the oil is black,  it may need to be changed. Or the bearings are over heating. Request a blower engineer Change oil every 6 months. All belts should be changed if one breaks. Replace belts every 24 months
Diffusers Walk around the system to visually check that all diffusers are working and that the aeration pattern is even

Remove any diffusers that are giving a poor bubble pattern and clean. The diffusers should be fizzing with no obvious signs of large bubbles.

Remove diffusers and brush clean once every 1 to 4 weeks

Diffusers in lagoon 1 may need a phosphoric acid clean every 6 months

Diffuser may need to be replaced after 5 years.

Air ring main Walk around the pipe once each day, listen for leaks or water noise. Condensate can accumulate in the pipe and restrict air flow. A 2mm hole may need to be drilled into underside of the pipe at locations where water collects. This is usually the lowest point at the furthest distance from the blowers  

3.6.  Chemical monitoring regime

In order to monitor the leachate treatment system and to fine tune the performance of the system, analysis of the raw leachate and the quality of the lagoons, tank and discharge water is required.

Table 3.6.1 Chemical analysis sampling frequency.

  Raw Leachate Lagoon 1 Lagoon 2 Discharge tank After discharge channel
Ammonium 1 - 7 1 - 7 1 - 7 1 - 7 1 - 7
Nitrate 7 - 30 7 - 30 7 - 30 7 - 30 7 - 30
BOD 30 30 30 30 30
COD 7 - 30 7 - 30 7 - 30 7 - 30 7 - 30
Suspended solids 0 7 7 7 7
PH 1 - 7 1 - 7 1 - 7 1 - 7 1 - 7
Alkalinity as CaCO3 1 - 7 1 - 7 1 - 7 1 - 7 1 - 7
Phosphate as PO4-P 0 1 - 7 1 - 7 1 - 7 0
suspended solids 0 7 7 7 0

1 =                   sample every working day

7 =                   sample every 7 days

30 =                 sample once a month

7 – 30 =           sample every 7 days until a pattern is established, then it can be reduced to once a month

The information gathered from the water analysis schedule should be stored on a spread sheet. Decisions regarding the chemical dosing and operation of the system are the based on the results. The analysis can be performed using test strips and the PalinTest colorimeter. Periodically samples should also be sent to a certified laboratory for checking.

Note.

Nitrate is produced as an end production of ammonium nitrification.  The concentration of nitrate in the leachate water therefore gives a measure of the split between heterotrophic assimilation and nitrification of ammonium.  If there is no nitrate then all of the ammonium removal is by assimilation, this is likely to be the situation during the winter months, hence the reason for molasses as a food source for the heterotrophic bacteria.

3.7  Chemical dosing requirements

The average figures for the raw leachate are as follows;

  • Ammonium 1000 mg/l
  • COD 1400 mg/l
  • Phosphate 5 mg/l
  • Alkalinity 5000 mg/l

Table 3.7.1 Chemical dosing requirements based on leachate water quality

Chemical dosing requirements and actions
Parameter value location Action 1 Action 2
pH 7.0 to 7.6

Lagoon 2

&

discharge tank

Add caustic if below pH 7.0, do not exceed pH 7.6. The dosing pump may be used to perform this operation.  By maintaining the pH of Lagoon 2 , there may be sufficient residual alkalinity to maintain the pH in the discharge tank. If the pH falls below 7 in the discharge tank, it will also require caustic dosing.  50 to 200 litres per day of caustic will be required. Avoid sudden changes and pH fluctuations Calcium carbonate or magnesium oxide granules can be added to lagoon 2 and the discharge tank. You can add up to 10 tonnes of this material because it dissolves slowly in response to the pH. The chemicals will reduce or negate the need to dose caustic
phosphate <10 mg/l Discharge channel Add phosphoric acid to lagoon 2 at a rate of 10 to 25 litres per day per 100 cubm of leachate treated Gradually increase the level of chemical dosing until the concentration starts to rise in the discharge water. There is no discharge consent for phosphate, however do not allow the level to exceed 10mg/l.  The phosphoric acid can be dosed into lagoon 2.
Oxygen < 3mg/l Lagoon 1  Stop addition of leachate to allow oxygen levels to increase.  This is automatic, however make sure the oxygen meter is working properly. Clean air diffusers, check air blowers are working properly
Ammonium >80 mg/l Lagoon 2 Make sure oxygen is above 3mg/l, pH is between pH 7.0 and 7.8. May require more phosphoric acid. If water temperatures are below 8 deg C, increase the molasses level. If the water quality in Action 1 column is okay, bacterial biomass may need to be added. Reduce leachate flow to try and achieve 80mg/l  before increasing flow rate.
>25mg/l Discharge tank Same as above Same as above
Biomass <1000 mg/l Lagoon 1, 2 and tank If the biomass levels are falling and start to approach 1000 mg/l. Additional sludge may need to be added to lagoon 1 & 2 In order to avoid the loss of activated sludge, NoPhos may be used at 1% of the sludge weight once or twice a year
NoFloc        
temperature < 8 deg C Lagoon 1 & 2 and discharge tank May require molasses addition at 50 to 150 litres per day for every 100 cubic metres of leachate treated Leachate flow may need to be reduced. See molasses note below
Molasses

As the water temperature drops, autotrophic bacterial activity decreases, and the alkalinity values as well as ammonium will increase.  Under these conditions it may be necessary to start adding or to increase the amount of molasses added to lagoon 2. The molasses acts as a food source for the more temperature tolerant heterotrophic bacteria.  These bacterial assimilate the ammonium and convert it into protein.  Because the COD is low in the leachate water, extra food will be required for the bacteria, especially during the winter months in order to remove the ammonium. Molasses should be added to the system at a rate of 50 to 150 litres per day, per 100 cubic metres of leachate treated. Alternatively it may be acceptable to add 1 cubic metre directly to lagoon 1 once a week

Measure the BOD/COD of the discharge leachate, if the levels start to increase after the addition of the molasses you may have to slow down or stop the addition.  Conversely if you are not complying with the ammonium discharge criteria, keep increasing the molasses dosing until the BOD/COD levels start to increase. The 50 to 150 litre/day figure given is a guide level, the actual quantity required may fall out-with this band.


 4.        Appendix

Appendix 1   Oxygen transfer and solubility in water

Table.Oxygen transfer efficiency of air diffusers

% oxygen already in solution Depth of water in aeration tank (meters)
1 2 3 4 6 30
5% 30% 35% 40% 45% 50% 95%
10% 25% 27% 30% 35% 40% 90%
20% 20% 22% 23% 25% 30% 85%
30% 15% 17% 18% 20% 22% 80%
40% 10% 12% 14% 17% 18% 75%
60% 6% 7% 8% 12% 15% 60%
80% 2% 3% 6% 7% 9% 45%
100% 0% 0% 0% 0% 0% 0%

Table  Concentration of oxygen in mg/l  equating with 100% oxygen saturation in water.

T deg C 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
0 14.60 14.65 14.52 14.48 14.44 14.40 14.36 14.33 14.29 14.25
1 14.21 14.17 14.13 14.09 14.05 14.02 13.98 13.94 13.90 13.87
2 13.83 13.79 13.75 13.72 13.68 13.64 13.61 13.57 13.54 13.50
3 13.46 13.43 13.39 13.36 13.32 13.29 13.25 13.22 13.18 13.15
4 13.11 13.08 13.04 13.01 12.98 12.94 12.91 12.88 12.84 12.81
5 12.78 12.74 12.71 12.68 12.64 12.61 12.58 12.55 12.52 12.48
6 12.45 12.45 12.39 12.36 12.33 12.29 12.26 12.23 12.20 12.17
7 12.14 12.11 12.08 12.05 12.02 11.99 11.96 11.93 11.90 11.87
8 11.84 11.81 11.78 11.76 11.73 11.70 11.67 11.64 11.61 11.58
9 11.56 11.53 11.50 11.47 11.44 11.42 11.39 11.36 11.34 11.31
10 11.28 11.25 11.23 11.20 11.17 11.15 11.12 11.10 11.07 11.04
11 11.02 10.99 10.97 10.94 10.91 10.89 10.86 10.84 10.81 10.79
12 10.76 10.74 10.72 10.69 10.67 10.64 10.62 10.59 10.57 10.55
13 10.54 10.50 10.47 10.45 10.43 10.40 10.38 10.36 10.34 10.31
14 10.29 10.27 10.24 10.22 10.20 10.18 10.15 10.13 10.11 10.09
15 10.07 10.04 10.02 10.00 9.98 9.96 9.94 9.92 9.89 9.87
16 9.85 9.83 9.81 9.79 9.77 9.75 9.73 9.71 9.69 9.67
17 9.65 9.63 9.61 9.59 9.57 9.55 9.53 9.51 9.49 9.47
18 9.45 9.43 9.41 9.39 9.37 9.36 9.34 9.32 9.30 9.28
19 9.26 9.24 9.23 9.21 9.19 9.17 9.15 9.13 9.12 9.10
20 9.08 9.06 9.05 9.03 9.01 8.99 8.89 8.96 8.94 8.92
21 8.91 8.89 8.87 8.86 8.84 8.82 8.81 8.79 8.77 8.76
22 8.74 8.72 8.71 8.69 8.67 8.66 8.64 8.63 8.61 8.59
23 8.58 8.56 8.55 8.53 8.51 8.50 8.48 8.47 8.45 8.44
24 8.42 8.41 8.39 8.38 8.36 8.35 8.33 8.32 8.30 8.29
25 8.27 8.26 8.24 8.23 8.21 8.20 8.18 8.17 8.16 8.14
26 8.13 8.11 8.10 8.08 8.07 8.06 8.04 8.03 8.01 8.00
27 7.99 7.97 7.96 7.94 7.93 7.92 7.90 7.89 7.88 7.86
28 7.85 7.84 7.82 7.81 7.80 7.78 7.77 7.76 7.74 7.73
29 7.72 7.70 7.69 7.68 7.66 7.65 7.64 7.63 7.61 7.60
30 7.59 7.57 7.56 7.55 7.54 7.52 7.51 7.50 7.49 7.47

 

Appendix 2   capacity of the system to treat leachate

Air flowrate is 1600 cubic meters of air per hour, how much leachate can the system treat in a lagoon based treatment process, given ammonium level of 1400mg/l and COD level 2000mg/l

Air flow per diffuser = 10 cubm/hr, No. of diffusers = 1600/10 = 160

Oxygen from each diffusers = 10 kg, total oxygen per day = 1600kg, per day

2 kg of oxygen are required per 1 kg of COD and 5 kg of oxygen is required per 1 kg of ammonium.

Oxygen demand of system per cubic meter

COD = 2000mg/l = 2 kg per cubm x 2 = 4 kg of oxygen is required

Ammonium = 1400mg/l = 1.4 x 5 = 7kg

Total oxygen demand = 7kg + 4 kg = 11kg

The amount of water the system can treat = 1600kg (oxygen from diffusers) / oxygen demand 11kg = 145 cubic meters per day


Appendix 3 Maintenance of the dissolved oxygen probe.

Probe maintenance

The oxygen probe requires very little attention, the degree of attention will depend upon the water type in which the probe is immersed. For most applications we recommend that the probe be removed from the water once a week and the membrane cleaned with a soft clean cloth. The breather hole on top of the probe should also be cleaned using a pin. This is all that you need do with the probe on a regular basis.

In the event that the membrane is damaged, the readings will become very erratic. Under these conditions the membrane should be replaced.

Membrane Replacement

7      The following procedure describes membrane replacement.

8      Remove the oxygen probe from the water and clean with a cloth or paper towel

9      Unscrew the bottom end cap, please note that the electrolyte containing a white deposit of colloidal solution of zinc oxide may be captained in the cap.   Discard this solution.

10   Using a coin, unscrew the threaded retaining ring in the membrane cap, remove and discard the membrane and small `o` ring located below the membrane

11   Clean the inside of the membrane cap thoroughly with a damp cloth, and finish off with a clean dry cloth.

12   Insert a new `o` ring, and then the membrane on top of the `o` ring. Screw down the threaded retaining ring until you feel tension, then give is a further 1/4 turn. If the membrane wrinkles, you will need to replace the `o` ring and membrane and try again.

13   Clean the inside of the probe top, you may clean the silver cathode with 1500 grade paper, take care as damage to the silver cathode can affect probe readings.  You can clean the angular zinc anode with rough abrasive paper in order to remove any oxidation layer. Clean the probe in freshwater to remove any of the fines.

14   Fill the membrane cap with electrolyte, and holding the probe vertically, screw the membrane cap on to the top of the probe, making sure that the large `o` ring is in place. Slowly screw up the cap, the excess electrolyte will escape though the breather hole.  Take care that you do not screw the cap on too quickly since this will stretch the membrane. Screw up the cap until it seals on the large `o` ring a then give it a further 1/4 turn.

15   Immerse the probe into the water, after 24 hours perform a calibration as per the instructions above.

diffusers etc.


The suppression circuit is used to drop the out-put of our oxygen probes from approximately 80mv to around 25mv, the component is fitted to the terminal strip at the back of the Eutech oxygen controller. 

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