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As a consequence of continuous increase of energy demand the energy price has seen a significant rise. Therefore the operational cost in which the electricity bill occupies the lion's share has come to concern for stakeholders in wastewater industry. Apart from treatment plants being safe and efficient, it is necessary to be more economic too.
Recent decades have seen tighter regulations regarding the effluents from treatment plants. For these purpose and other factors such as population boom on the existing treatment plants, different advances in treatment plants have been invented. One of the developments is the inclusion of media in the activated sludge process (ASP) to enhance the removal efficiency of BOD, COD and Nitrogen. These can be achieved through growing biofilm on the supporting carriers in the aeration tank. This research proposal would look at the effect of media inclusion in ASP on the energy reduction.
Prior to identifying the main parameters that influence reducing energy consumption in the ASP containing packing media as biofilm supporter, one has to be familiar with packing materials in the ASP in which the materials may be fixed in the aeration tank or suspended in the mixed liquor (Metcalf and Eddy, 2003). Integrated Fixed film Activated Sludge processes (IFAS) is defined by Rosso et al. (2011) as the mixture of biofilm reactors and the ASP by inserting and retaining of biofilm carrier media in activated sludge reactors. Moving Bed Biofilm Reactor (MBBR) is described as a process in which materials of small cylindrical shapes carrier elements is added into the aeration tank (Metcalf and Eddy, 2003). Depending on the pollutant removal both processes are employed more widely where the footprint is constrained and/or when the upgrade is required for existing conventional activated sludge plants due to the increase of population (Wang et al., 2005 cited in Rosso et al., 2011).
Firstly, regarding the energy consumption, nutrient removal and Oxygen transfer and uptake, Rosso et al (2011) conducted a research on T.Z. Osborne Water Reclamation Facility treatment plant in Greensboro, NC (USA) with the capacity of 15,000 m3/d. Using off-gas analysis the research equipped an operation of independent full-scale IFAS reactors with course bubble aeration and AnoxKaldnes media in parallel to an existing ASP. The goal of the research was to treat two different influent flows which reached to 33,700 m3/d in January and 34,100 m3/d in June of 2010. Having compared the two processes using the same environmental conditions and pollutants loading, the author doubled the influent flow entering the IFAS unit. After operating the two processes, it was concluded that by having the same removal efficiency regarding COD and Ammonia, IFAS is a feasible process for the expansion required by the increase of influent flow especially where there is no enough land space. With respect to IFAS, it is also resulted that due to the use of elevated air there was an increase in oxygen demand removal. Furthermore the energy consumption by air blowers in IFAS to keep the media mixed and aerated exceeded the ASP by a factor of 2. However the author failed in considering two points; firstly, testing different filling ratio (FR) of the media to the aeration volume which is an important factor to be taken in consideration and secondly the impact of IFAS process on energy use in the secondary clarifier and sludge processing because apart from being IFAS efficient in improving treatment level of Conventional Activated sludge (CAS), it has a greater stability process (Sriwiriyarat et al., 2008 cited in Rosso et al., 2011) and less sludge production and reduces the solid loading on the secondary clarifier (Stricker et al., 2009 cited in Rosso et al., 2011).
Secondly, Kappel (2009) has conducted another research on Henley sewage treatment work in South East of England, where the plant upgraded by Thames Water from CAS to IFAS to achieve the higher quality of the effluent without constructing new units for the treatment plant. The research aimed to estimate energy consumption for IFAS through oxygen transfer efficiency and compare it with CAS. For IFAS (Prosil®) as packing media was introduced to the aeration tank to grow biofilm for the purpose of achieving nitrification to the consent level. Furthermore fine bubbles were used for the aeration and off-gas technique was deployed to measure oxygen transfer rate. The results indicated that at zero dissolved oxygen concentration SOTE per meter ranged between 3.52 to 6.81%, causing oxygenation efficiency from 1.08 to 3.17 kgO2.kWh-1. Moreover it was found that COD, BOD5 and Ammonia loading does not affect SOTE, higher mass transfer coefficient was recorded due to the increase of airflow rate and at dissolved Oxygen below 3.0 mg.L-1 the IFAS process was more efficient than CAS which means that IFAS is not more energy intensive than CAS. However the author did not estimate the impact of IFAS system operation on the secondary clarifier and sludge processing which might cause low loading and consequently lower energy consumption. Lastly the author did not consider the effect of filling ratio of media to the aeration tank.
In a nutshell, as it is discussed above there are different approaches to estimate the impact of media inclusion on energy reduction in ASP, both Rosso et al (2011) and Kappel (2009) experimental studies have applied the off-gas technique to analyse the SOTE in the mixed liquor in both CAS and IFAS. It is considered that off-gas technique is an ideal method to determine the oxygen mass transfer and it has been developed for the first time in a full-scale plant by Redmon et al. (1983) (Rosso et al., 2011). However, they are different in employing bubble size for the aeration. Kappel (2009) has used fine bubbles which gave good results in lowering the energy used for the aeration. So by deploying off-gas, using (Prosil®) as packing media for IFAS, and different FR might come with acceptable results regarding energy reduction for the aeration in MBBR or IFAS processes.
Aims and Objectives
The aim of this proposal is to obtain basic SOTE in IFAS systems and from this estimate the energy requirement in comparison to CAS. In addition the impact of the different filling ratios of carrier and hydraulic loading rate on the IFAS process in a full-scale plant to see if the changing conditions affect the energy footprint reduction
Research design and methods
As it is described in Rosso et al (2011), the treatment plant of T.Z. Osborne Water Reclamation Facility in Greensboro, NC (USA) is originally ASP with the capacity of 15,000 m3/d, but due the increase of hydraulic loading the current situation of the plant requires an upgrade to meet the new influent flow with certain COD and ammonia concentration. This research proposal suggests the following experimental approach:
T.Z. Osborne Water Reclamation Facility WWTP has 4 aeration basins; one of the basins will be used as CAS process and the other 3 parallel basins as IFAS processes. The influent flow will be divided into 4 basins as each IFAS basin receives twice the flow of the parallel CAS. Fine bubble diffusers will be used for both CAS and IFAS process and the IFAS basins will be filled at different filling ratio of carrier media (Prosil®). The filling ratio will be divided into three ratios so that aeration tank no.9, 10 and 11, would be filled with carrier of filling ratios of 30%, 40 and 50% respectively. The out flow from CAS aeration tank no. 12 will be discharged to final clarification no.7 and the other out flows from IFAS from aeration tank no.9, 10 and 11 will be discharged to final clarification no.7
The off-gas analysis will be performed in accordance to US EPA standard protocol for testing in process water (US. EPA, 1989) , it should be based as the original method which is developed by Redmon et al. (1983) to estimate the oxygen transfer efficiency of the of fine bubble diffusers of the tanks aerated by air blowers in the water column underneath the a floating off-gas collection hood. The tests will be performed in June 2013 and January 2014 to observe the worse cases of both warm and cold waters. Thus the efficiency of air blowers will be determined and lastly the energy footprint can be estimated. The following gantt chart illustrates the sequences and time management for this research.
Data and Analysis
In order to find the energy consumption in the aeration system, it is necessary to find the efficiency of air blowers by calculating the rate of oxygen transferred to the mixed liquor which is considered as a basic parameter and defined as oxygen transfer rate (OTR) in (kgO2h-1)
(kgO2h-1) Henze (2008)
KLa = Liquid-side mass transfer coefficient (h-1)
DO = dissolved oxygen in water (kgO2m-3)
DOsat = dissolved oxygen in water at saturation (kgO2m-3)
V = Water Volume (m3)
OTR is a direct measurement of aeration capacity in the system. So the relationship between OTR and aeration efficiency (AE) is defined as below:
(kgO2/kWh) Henze (2008)
Where P = power drawn by the aeration system (kW)
Because of site-specific environmental and process conditions, standard conditions should be used which are defined as zero salinity, zero DO and 1 atm), hence the oxygen transfer efficiency becomes Standard Oxygen Transfer Efficiency (SOTE, kgO2h-1) and the aeration efficiency is described as Standard Aeration Efficiency (SAE, kgO2/kWh). Furthermore the mixed liquor contains water, dissolved and suspended contaminants and it makes a deviation in the performance of the aerator, so it is important to find factor which the ratio of process to clean water mass transfer coefficients:
or Henze (2008)
SOTE = oxygen transfer efficiency at standard condition (%),
αSOTE = oxygen transfer efficiency in process water at standard conditions except for the effect of contaminants on the mass transfer coefficient (%).
Because each IFAS tank receives twice the influent flow of CAS therefore the oxygen demand (OD, kgO2h-1) would be calculated as:
Rosso et al.(2011)
âˆ†COD = COD oxidised in the aerobic reactor (mgl-1)
âˆ†COD = ammonia oxidised in the aerobic reactors (mgl-1).
And P power drawn by the aeration system can be calculated throw the following equation:
Rosso et al. (2011)
ρair = air density (kgm-3)
Atank = tank bottom area.
Jair = Air flux (m s-1)
R = universal gas constant (8.314 J mol-1 K-1)
T = ambient temperature (K)
C = molecular weight of air (kg kmol-1)
Æž = combined motor and blower efficiencies.
Z = hydrostatic pressure corresponding to diffuser submergence
hL = head loss in the air distribution line (Pa)
DWP = dynamic wet pressure (pressure drop across the diffusers, Pa)
Pi = inlet pressure (Pa)
Lastly the energy consumption of each filling ratio of IFAS tanks and CAS tank will be calculated and compered to find out which filling ratio will has the impact of energy reduction.
In order to carry out this research, it is important to asses some risks that may affect the results or delay the process.
The first risk that may hinder or delay this research is the mechanical breakdown of the equipment such as pumping system or air diffusers. To control such risk it is necessary to have regular and continuous maintenance and check the equipment prior starting the process. In addition to employ skilled operators and make sure they are trained will to fix any problems caused by mechanical issues.
The second likely risk may be due to human mistakes and errors during the data collection, because of having many experimental data it would make difficult to be sorted and organised. This risk could be reduced or controlled by being the data organised by skilled staff and kept properly, additionally creating a backup of all recorded data and regular checking and monitoring data processing will lead to identify unusual results.
The third risk will be come from financial problems due to the sponsor's change of interest and natural disasters that may happen.