Cost-effective odour control in pressurized sewers
What are the options for controlling odour during sewage pumping? Gary Hoyland of Mott MacDonald details the reasons why a dual dosing regime, using both oxygen and nitrate dosing, has been employed in Hong Kong.
The pumping of sewage through long pressurised sewers has become far more common in modern schemes, particularly those serving existing communities in congested areas where environmental regulations, space constraints or cost effectiveness make it impossible to build a WwTP at the gravitational low-point.
Whether a particular H2S concentration in sewage will cause odour nuisance depends on factors such as the amount of turbulence in the sewage, the pH value of the sewage and the nature of the environment around the emission. Generally, a dissolved H2S concentration of 0.25mg/l is unlikely to cause significant odour nuisance, whereas a concentration of 2mg/l will almost certainly do so.
The rate of production of H2S (g/m3 of sewer per hour) in septic, full sewers varies with sewer diameter and sewage strength. To determine H2S concentration in the sewage, the production rate is multiplied by the average retention time (hours) of the sewage in the sewer. For a 400mm sewer and a sewage BOD of 300mg/l, the predicted total production rate of H2S is 3.5g/m3 of sewer per hour. Assuming an average flow velocity of 1m/s, this rate is equivalent to a H2S concentration increase of 1.0mg/l per 1000m of sewer. Thus, even comparatively short pressurized sewers can give rise to odour nuisance.
In addition, the rate of H2S production in the slime is generally greater than the production rate in the bulk sewage for sewers with diameters less than 500-600mm, and less for larger sewers. The consumption rate of dissolved gaseous oxygen in any particular full sewer, under aerobic conditions, is about a factor of ten higher than the production of H2S in the same sewer under septic conditions.
Main odour control methods
The five main odour control methods currently in use are:
- Dosing air into the sewer immediately downstream of the pumping station.
- Dosing gaseous pure oxygen into the sewer immediately downstream of the pumping station or, for sewers longer than about 5km, at a downstream position.
- Dosing nitrate solution into the sump at the pumping station.
- Dosing a solution of an iron salt into the sump at the pumping station or near the sewer outlet.
- Installing ventilation equipment at the sewer outlet.
Some methods may be combined. For example, iron dosing and ventilation are commonly combined. Mott MacDonald has successfully combined oxygen dosing with nitrate dosing, as described in the Hong Kong case study (see below).
The unit costs (£/m3 of sewage) of the main odour control methods depend on factors such as the scale of the operation, the nature of the sewage, the horizontal profile of the sewage and the local cost of the chemicals. For medium and large applications of oxygen injection, iron dosing and nitrate dosing, the costs of the respective chemicals usually dominate the overall costs.
For medium and large applications, the comparative chemical costs for septicity control usually conform to the following order:
Air dosing < Oxygen dosing ÷ Iron dosing < Nitrate dosing
Air injection is potentially the most cost-effective method. However, it has many disadvantages and is not technically viable for most applications. There are only a few applications of air dosing around the world and little performance data is available.
The cost of nitrate dosing is high owing to the high cost of the nitrate chemical. The international price of nitrate oxygen, supplied as calcium or sodium nitrate solution, is about £1.00/kg of oxygen. By comparison, the cost of liquid oxygen varies from about £0.15 to £0.30/kg, or more, depending on the location and the consumption rate. The reaction rate of dissolved gaseous oxygen with sewage is about twice as high as that for nitrate oxygen, but even taking this difference in reaction rate into account, the cost of controlling septicity by nitrate dosing is roughly twice that of controlling by oxygen dosing.
That said, at small applications, nitrate dosing can be more cost-effective than oxygen injection owing to the comparatively high cost of the oxygen storage and injection equipment. Nitrate dosing’s popularity is also due to its ease of use, reliable performance and safe operation.
Iron dosing is reasonably cost-effective but merely moderates rather than eliminates odours. This limitation arises because, although the iron may react with the H2S, it does not react with most of the obnoxious organic substances produced in sewers. Iron dosing is attractive when coupled with a ventilation and scrubbing system at the end of the sewer to prevent the release of residual odours.
Hong Kong: controlling septicity
Mott MacDonald has found that the cost of septicity control can be reduced by combining methods.
Sewage discharged from Hong Kong’s new airport and the North Lantau Development Area flows into the pumping station at Tung Chung.
From the pumping station it is pumped through a 6.06km sewer, 1100mm dia, to the WwTP at Siu Ho Wan. In the absence of preventative measures, the sewage turns septic in the sewer, producing odour levels at the treatment works that prevent normal operation.
The maximum design flow rate for the sewer is 95,000m3/day. However, flow rate was only 5000m3/day at commissioning in February 1998, increasing to 11,000m3/day after the new airport opened in summer 1998.
Flow rate is expected to reach 30,000m3/day by 2001 and 60,000m3/day by 2010 as the airport and the Development Area develop further.
The method of odour control at Tung Chung involves the dosing of pure gaseous oxygen (from the evaporation of liquid oxygen) into the sewer immediately downstream of the pumping station and calcium nitrate solution dosing into the sewage immediately upstream of the station. The available oxygen content of the calcium nitrate solution is 18 w/w%.
Dual dosing is needed because it is not currently possible to dose and dissolve sufficient oxygen to prevent the onset of septicity in the sewer. When the flow rate of the sewage reaches about 35,000m3/d in 2001, it will be possible to dose sufficient oxygen and the use of nitrate dosing will end. Until then, both pure oxygen and nitrate oxygen will be dosed, rather than nitrate alone, to reduce costs.
The local unit costs of pure oxygen and nitrate oxygen are HK$4.06/kg and HK$14.70/ kg respectively. Although dissolved oxygen reacts about twice as fast as nitrate oxygen, it remains the more cost-effective of the two chemicals. The overall objective of the control system is to use pure oxygen to satisfy the oxygen demand of the sewer as much as is possible and to use nitrate oxygen to satisfy the residual demand.
Dissolved oxygen inhibits the biochemical uptake of nitrate oxygen. Thus, dissolved gaseous oxygen is consumed in the upstream section of the sewer and nitrate oxygen in the downstream section. To prevent septicity, a small nitrate residual of about 2mg/l is needed at the outlet.
Where sufficient oxygen cannot be dosed at a single point, and subsequently dissolved, the option of dosing at several points along the sewer is a possibility. In Mott MacDonald’s Hong Kong work this alternative was rejected because there are no suitable dosing sites other than at the pumping station.
Operational data show the relationship between the nitrate dose at the pumping station and H2S concentration in the sewage at the sewer outlet on days when the flow rate of the sewage was between 10,000 and 13,000m3/d and the dose of pure oxygen constant at 65mg/l. A dose of nitrate solution of 0.7 to 0.8l/s has been sufficient to eliminate the H2S concentration consistently. However, to reduce operating costs, the nitrate dose has been set at 0.6l/s on the basis that occasional, minor odour nuisance is tolerable.
Pure oxygen and nitrate dosing will vary with sewage flow rate, altering the cost of the odour control. For example, in spring 1999, when sewage flow rate was about 12,000m3/d, the chemical costs amounted to HK$28,000/d, equivalent to HK$2.3/m3. In 2010, when the flow rate is expected to reach 60,000m3/day, the daily cost will reduce to HK$11,700. This reduction will be due to the change from nitrate/oxygen use to pure oxygen. Oxygen consumption will increase as nitrate is reduced.