Published on WWT Online May 2020
Rowland Minall explains the techniques available to inform and improve aeration system design and operation including oxygen transfer testing.
Wastewater treatment is extremely energy intensive with energy demand varying on a site by site basis between 2-20kWh/Population Equivalent. Some of this energy is associated with pumping but often the bulk of the energy and associated CO2 emissions (up to 60%) are associated with aeration for secondary treatment in the Activated Sludge Process (or a variant thereof).
Fine Bubble Diffused Aeration (FBDA) is the mainstay of many biological wastewater treatment and liquor treatment plants constructed in the UK over the past twenty years. The premise is based upon placing a diffuser membrane at the base of reactor (normally up to 7m deep) and forcing air through the membranes into the water being treated using blowers.
This equipment is typically installed during capital schemes involving replacement or refurbishment of assets and once submerged are difficult to inspect without emptying operational tanks or lanes.
Each individual diffuser system provider will provide warranties for oxygen transfer efficiency on each of their products or installations, often on a site-specific basis. These are routinely based against a given oxygen demand and resultant airflow for a prerequisite design horizon (15 to 20 years). Whilst designers and fabricators seek to provide the most efficient solutions, once installed, direct routine inspections are difficult to achieve. The oxygen transfer efficiency of the overall system drives the amount of air that is needed to meet the oxygen demand, so that lower efficiencies mean that more air (and more energy) is required to meet the oxygen requirements of the system.
When performed correctly, off-gas testing can prove to be a useful tool for measuring aeration system and diffuser condition without emptying aeration lanes. It involves collecting direct measurements of oxygen transfer efficiency (OTE) across carefully selected monitoring positions to give a representative understanding of OTE across the whole aeration lane. In addition to assessing existing aeration asset condition, results can also feed into standard equations to inform design.
Aeration System Sizing and Design
The major components of an aeration system are the blowers which force air into an aeration manifold and the diffusion array, which diffuses air from the manifold into the water being treated. Sizing of the aeration system capacity including blower size and diffusers starts at the beginning of a project with a design horizon of up to twenty years. The criteria used to evaluate the size are;
- Incoming contaminant load and biomass under aeration
- Site specific factors, including elevation above sea level, operating temperature and two key wastewater specific measurements, the alpha factor and the beta factor
- Sizing the blowers must account for pressure drops along the manifold and the depth of water being aerated but fouling of the diffusers over must time must also be accounted for
Standard engineering formulae can be applied to values determined for the site such that the aeration system design can be undertaken.
Alpha Factor
The alpha factor is the ratio of mass transfer coefficients (KLa) for oxygen into the test water and the clean (potable mains) water. In wastewater treatment plants especially, this value can change throughout a wastewater treatment lane, especially in plug flow reactors as the dissolved contaminant materials are consumed.
Although Alpha factors can be assumed to be between 0.7 and 0.9 (dimensionless), dependent upon the position within an aeration lane, testing KLa’s can readily be undertaken within a controlled laboratory environment according to methods described by the American Society of Civil Engineers ‘Measurement of Oxygen Transfer In Clean Water’. To test the KLa, waters are deoxygenated using nitrogen gas, then reaerated using atmospheric air. Logging the dissolved oxygen trend over time in both clean water and site wastewater, allows a site-specific alpha factor to be calculated from the rate at which the sample reaerates.
This same technique (with minor modifications) can also be used to measure the capacity and efficiency of newly installed aeration systems, often required as part of contractual takeover requirements on new assets.
Beta Factor
Dissolved solids within a water stream affect the ultimate solubility of oxygen within the water. This has less impact than the Alpha factor, generally between 2 and 5% of oxygen demand, however this can be especially problematic at coastal sites with saline intrusion or on liquor treatment plants treating solutions with high dissolved solids concentrations.
Comparing the maximum oxygen solubility of a filtered sample of potable water allows the Beta Factor to be readily ascertained.
Fouling Factors
Many modern FBDA systems are designed with self-cleaning in mind, with control systems allowing for temporary depressurisation resulting in shearing off any biomass or other foulants that may be present. Less easy to remove are scalants, that may form within the membrane pores which can cause blockages, reduce the oxygen transfer efficiency and increase the backpressure required by the blower to successfully deliver air into the process water. Design Fouling Factors are typically in the range of 0.7 to 0.9 but these will change over time as the equipment is used.
Increased or excessive fouling may result in inefficient aeration and operation at low dissolved oxygen concentrations such that anoxic conditions may become prevalent. At these levels treatment may be compromised and the risk of emission of nitrous oxide, with a greenhouse gas equivalent 300 times greater than CO2, increases.
Fouled diffusers can be cleaned in-situ, dependent upon the condition of the units, completed by pumping small volumes of weak organic or carboxylic acids into diffuser manifolds. The cleaning of diffusers increases asset life, reduces operational backpressure (reduces energy demand on the blowers) and can improve the transfer of oxygen into the process water.
Oxygen Transfer Efficiency Measurement
Measuring the Oxygen Transfer Efficiency in an operational lane allows the actual oxygen transfer efficiency to be evaluated which provides an indication of diffuser condition and suitability resulting in the evaluation of airflow. This can be used to target individual areas which may need more attention than others within a bank of aeration lanes. Furthermore, this may be evaluated live and in lane using a floating buoy and an oxygen sensor arrangement, a technique which can readily be adapted to assess other gaseous emissions such as Nitrous Oxide.
To learn more about the Aqua Enviro OTE assessment, alpha factor and beta factor testing, diffuser cleaning, carbon efficiency, site audit services or any other ways in which we can help, please contact Paul Pickard
paul.pickard@aquaenviro.co.uk
T. 01924 242255
