University of Waterloo
Department of Civil Engineering
EnvE 472 Wastewater Treatment
Activated Sludge Experiment
Introduction
Activated sludge is the most common biological process employed to treat municipal
wastewaters. It is also utilized for many industrial wastewater treatment applications. In
the activated sludge process, organics that are present in the wastewater are either
oxidized into carbon dioxide and water or assimilated into microbial biomass. Inorganic
nutrients such as ammonia may also be oxidized. The defining features of the activated
sludge process are:

the use of a biomass recycle to increase the biomass concentration in the aeration
tank.

biomass removal from the system is independent of the hydraulic flow to the system.
Incorporation of these features into the activated sludge process provides flexibility in
system operation to account for changes in wastewater flow, concentration,
temperature, etc. In this laboratory, a batch experiment will be performed to assess the
process response and to obtain information for the calibration of biokinetic models.
Materials
The biomass that will be employed in this lab is being produced by two pilot scale
sequencing batch reactors (SBRs) that are being operated at different solids residence
times at the New Hamburg wastewater treatment plant. Samples of the aeration basin
contents will be collected and transferred to bench scale batch reactors in the lab to
simulate the “react” portion of the SBR cycle.
The monitoring and analytical equipment that will be employed in this project include:
Chemical Oxygen Demand (COD) analyzer
suspended solids apparatus (filtration and ovens)
dissolved oxygen probe and meter
Method
The pilot SBRs are being operated in semi-continuous mode with differing solids
residence times. The solids residence time is manipulated by the operator by adjusting
the mass of biomass (measured as volatile suspended solids) that is removed from the
system (i.e. “waste”). The following describes the sequence of operations that is
employed to operate an activated sludge process in sequencing batch reactor mode:
Idle
Fill
React
Waste
Settle
Draw
The reactors repeat this cycle on 6 hour intervals. The reactors differ in the volume of
the reactor that is “Wasted” in each cycle. The reactors will have just gone through the
“Draw” immediately before the laboratory begins. Hence, the reactors can be
considered as being in the “Idle” period when the lab begins. Each group will be
provided with a sample of the “Waste” streams from the previous cycle. This should be
considered as being representative of the conditions that are present in the reactor at
the end of the previous “React” period and can also be used to calculate the mass of
biomass that is removed from the systems in each 6-hour cycle.
The following steps will be followed in this lab:
1.) A volume of wastewater will be added to each reactor at the beginning of the
experiment (“Fill”).
2.) The reactor contents will be sampled and analyzed for soluble COD (CODsol),
total suspended solids (TSS), volatile suspended solids (VSS) and dissolved
oxygen (DO) on hourly intervals for the first 3 hours of the “React” period.
3.) The soluble COD of the wastewater added to the reactor should be
characterized.
4.) The lab technician will provide each group with the volumes of wastewater that is
added to the pilot scale reactors in each cycle, the reactor volumes during
“React”, the volumes of biomass wasted and the volumes of treated wastewater
that is removed during the “Draw” period for each cycle.
The following sections describe the specific details required to complete this lab.
Suspended Solids Analysis
Suspended Solids analyses must be conducted on each sample from the bioreactors.
Properly prepared filters have been left in aluminum pans in the drying oven and should
be removed (pan & filter) when needed and placed into a desiccator and cooled for 5-10
minutes. When cooled, they should be weighed as a unit (pan & filter) on the analytical
balance to the nearest 0.1 mg (0.0001 g.). Record these weights on the lab. computer.
1. Perform in duplicate. Filter 10 mL. of sample from your bioreactor (use a
pipetter), collecting the filtrate BEFORE rinsing the filter.
2. When finished, place the filters and pans in the drying oven.
The weighing for Total Suspended Solids will be performed by the technician, as well as
firing and weighing for Volatile Suspended Solids, and these data will be tabulated on
the lab. computer within 48 hours.
Chemical Oxygen Demand, COD
Feed samples will be analyzed for COD without filtration while the filtrate collected from
the Suspended Solids analyses will be used for soluble COD analyses.
1. Measure 2.0 mL. of sample (use a pipetter), and carefully place into the
prepared COD reaction vials, with the opening of the vial pointed away from your
face, and then cap tightly. The amber liquid in the vial is a strong acidic solution
and extreme heating (exothermic reaction) of the contents will occur when mixed
with your sample.
2. Mix by, grasping the plastic screw-cap on the vial making sure that is has been
properly tightened, then invert the tube several times to mix your sample with the
acid reagents in the COD vial.
3. Place the mixed vial into the heating block for digestion at 150° C.
The digestion step, and the Colorimetric determination of COD will be performed by the
lab. technician and the data will be recorded on the lab. computer within 48 hours.
Suggestions for Data Analysis and Project Report
1.) Discuss the quality of the data by comparing duplicate analyses
2.) Compute the solids residence time (SRT) and the hydraulic residence time
(HRT) for the two reactors where:
SRT  V a X m
Qw X w
HRT  V a
Qw / w
Where: Va = aeration basin volume, mL
Xm = biomass concentration in aeration basin, mg VSS/L
Qw = daily volume of biomass wasted from the aeration basin, mL/day
Xw = concentration of biomass in waste stream, mg VSS/L
Qw/w = daily volume of wastewater fed to the reactor, mL/day
3.) Calculate sCOD removal efficiencies for the two reactors over the 6 hour cycle.
How are they affected by the solids residence time?
4.) Discuss the response of dissolved oxygen as compared to the COD response.