Predicting WWTP Sludge Production And Power Savings From Facultative Operation Of Aerobic Digesters

By R. Picket, President, Liquid Solids Separations, C. Proffitt, Newport Utilities, Wastewater Manager, and T. Mullenix, President, ENNIX INC

Objective

The objective of this white paper is the prediction of the wastewater treatment plant (WWTP) sludge production and subsequent power savings realized from operating the aerobic digesters via facultative digestion using technology from ENNIX⁽⁶⁾  INC. Development of the predictive spreadsheet permits one to demonstrate the power savings to potential users of the facultative digestion⁽⁶⁾ technology.

Overview

A spreadsheet was generated and compared against operational data from the Newport Utilities Wastewater Treatment Plant. Components of the spreadsheet are computations for determining atmospheric pressure at various altitudes and temperatures^(3,4), oxygen solubility at temperature and altitude⁽⁵⁾, modeling of the aeration system with a focus on the aerobic digesters⁽²⁾, and determination of the sludge age and observed yield coefficients (lbs VSS/lb CBOD₅ removed) from the operational data from January through July 2014.⁽¹⁾ The purpose of the spreadsheet is to enable prospective users of the facultative digestion technology⁽⁶⁾ to see what potential power savings might be realized.

Candidate Selection

With over 50 plants using the facultative digestion⁽⁶⁾ technology since 1999-2000, Newport Utilities WWTP was selected for this white paper because of a complex wastewater which is 60 percent industrial and 40 percent municipal that presents frequent challenges to operations because of a wastewater that can, on occasion, change characteristics very rapidly. Because of the nature of the wastewater numerous operational parameters are measured and accurately recorded in EXCEL spreadsheets enhancing the ability of development of the predictive spreadsheet. The WWTP has an elevated skilled workforce with 14 operational and other personnel who have their Tennessee Grade IV wastewater license; the highest license attainable in Tennessee. Newport Utilities WWTP is the “recipient of the Kentucky-Tennessee Water Environment Federation Wastewater Operational Excellence Award for having no more than one violation of its NPDES permit 14 of the last 15 years.”

Wastewater Description

The Newport Tennessee WWTP is a conventional activated sludge plant that treats a complex wastewater which is 60 percent industrial and 40 percent municipal. The industrial component consists of one food processing plant, one organic chemical manufacturing plant and two electroplating industries.

WWTP Description

The plant is a conventional activated sludge plant with a rated capacity of 4.35 MGD and an average daily flow of 2.8 MGD with design headworks loading of 30,350 pounds of biochemical oxygen demand (BOD)/day. The following table is data from January 2014 through July 2014 and is the data used in development of the spreadsheet.

Table 1:

Wastewater is pumped through the screening and teacup system (hydro-cyclones) into a selector blending basin where return activated sludge (RAS) is blended with processed primary wastewater. The combined flow then flows through the aeration basins to 4 center-fed conventional clarifiers. The overflow then reports to the nitrification tower for ammonia nitrogen removal (NH₃-N). The Nitrification tower overflow reports to sand filtration then to the chlorine contact chamber and finally to post aeration then to the receiving stream.  Facultative digested sludge⁽⁶⁾ is delivered to two 2.5 meter Komline Sanderson belt filter presses (BFP) for dewatering. Dewatered solids report to a Dragon Dryer, which produces a Class A biosolids that is sold to farmers.

Sludge Production Calculation From CBOD₅

Sludge production was calculated by generating the observed yield coefficient from operational data taken from the WWTP operational data, January through July 2014. The observed yield coefficient was determined from the following procedure

MCRT = 1/(Y*F/M*eff –Kd)⁽¹⁾

Where  MCRT    mean cell residence time days

                Y              Yield Coefficient----lbs CBOD₅ removed/lb MLVSS produced

                Kd           endogenous decay coefficient----lbs MLVSS removed per day/lb MLVSS under aeration              

                Eff          CBOD₅ removal efficiency through the aeration basin

The constants Y and Kd were determined using Microsoft Solver⁽⁷⁾.

Y_obs =1/(MCRT*F/M*Eff)⁽¹⁾

Where  Y_obs         Observed (aka net) yield coefficient---- lbs CBOD₅ removed/lb MLVSS produced

                F/M       Food to Microorganism ratio----lbs CBOD₅/lb MLVSS under aeration

Waste activated sludge (WAS) is pumped to the digesters from return activated sludge (RAS). To obtain pounds of Volatile suspended solids pumped to the digester ash content of the WAS is a routine operational parameter.

Clarifier solids inventory was not considered in these calculations.

Calculated Aerobic Digester Power Demand

At the Newport TN WWTP each of the 0.35 MG aerobic digesters have one row of Sanitaire bottom fitted course bubble 26” ss diffusers.  The power calculation⁽²⁾ for course bubble diffusion used 35 SCFM for mixing, α factor of 0.8 and the assumption that a stable sludge is reached at 38 percent VSS reduction.  The Newport TN WWTP plant has a dedicated 250 horsepower blower for the aerobic digesters enabling a comparison between the actual power consumption and the calculated power consumption see Table 1.  

Where:                 SCFM----standard cubic feet of air per minute

                                α---- ratio of oxygen transfer efficiency (OTE) in wastewater to OTE in tap-water

                          VSS-volatile suspended solids                       

To compensate for altitude and temperature in the spreadsheet, the Barometric Formula^(3,4) was used along with empirically derived formulas for dissolved oxygen⁽⁵⁾. 

Aerobic Digesters Operated In Facultative Mode

In June 2009, Newport TN WWTP turned off the air to the aerobic digesters and operated without air achieving facultative digestion⁽⁶⁾ without going anaerobic. Also no capital equipment modifications had to be made for the conversion. The digesters operate in a plug flow continuous decant fashion enabling a non-interrupted flow of biosolids to sludge dewatering. The dissolved oxygen stays close to 0.3 ppm even in the hottest months of the year. The table snapshot taken from August 2013 operating data spreadsheets illustrates the average dissolved oxygen for the digesters. Newport TN WWTP has not had odor or rising sludge problems since the operation began.

The facultative digestion process⁽⁶⁾ is achieved through the addition of a proprietary biochemical product⁽⁶⁾.

Table 2:

Note: The data in this chart reflect facultative operation of the aerobic digesters.

Aerobic digesters in facultative mode

Summary

The predictive spreadsheet for resultant power savings⁽²⁾ by the operation of aerobic digesters through facultative digestion⁽⁶⁾ compared favorably with actual data according to the Wastewater Department Manager of Newport Utilities WWTP. This calibration thus makes it possible to demonstrate to likely users of the technology, potential savings realizable by operating the aerobic digesters in a facultative mode.

Power consumption in the aerobic digester has two components, mixing and VSS reduction. Therefore, the prediction of sludge production in this white paper is necessary to estimate the total power consumption in aerobic digestion.

REFERENCES

1. EPA. (1977). Aerobic Biological Wastewater Treatment Facilities Process Control Manual., ii-16-ii-19
2. Bolles Steven, A., Modeling Wastewater Aeration Systems To Discover Energy Savings Opportunities. Process Energy Services, LLC. 1-11
3. Nave R., The Barometric Formula. http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/barfor.html .
4. Math24.net Differential Equations. Barometric Formula.  http://www.math24.net/barometric-formula.html
5. Sensa, Fred, copyright, 1997-2010, How Can I Predict Oxygen Solubility in Water, use of empirical formulas for dissolved oxygen as shown on page one of the URL., http://antoine.frostburg.edu/chem/senese/101/solutions/faq/predicting-DO.shtml,
6. Mullennix Ted, 1999-2014, ENNIX INC., Sustainability Through Biology, http://www.ennix.com
7. Microsoft Solver, http://office.microsoft.com/en-us/excel-help/about-solver-HP005198368.aspx