Case Study 2: Anaerobic wastewater treatment Drago Petruiu Lecturer: Tim Hendrickx

A waste water treatment plant receives the wastewater from the municipality 30 000 m3/d and

from an industrial site 1500 m3/d. The composition of the wastewaters is shown in the table 1. The

current treatment for the combined wastewaters is a pre-denitrification process combined with

chemical phosphorus removal. As the engineer of this wastewater treatment plant, you are asked to

investigate the possibility of treating the industrial wastewater separately in an anaerobic reactor.

The municipal wastewater will still be treated in the current process.

Table 1 Wastewater characteristics

Domestic Industrial

Flow m3/d 30 000 1500

COD mg/l = g/m3 521 3750

Total N mg/l = g/m3 48 154

Total P mg/l = g/m3 7 6

Temperature C 15 32

a) Calculate the composition (COD, N and P concentrations) of the combined waste waters.

Does the industrial wastewater contribute for a large part to the total COD, N and P load on

the current wastewater treatment process?

The calculations are done in the attached Excel file.

Table 2 and 3 summarizes the results and presents an overview on the industrial wastewater

contribution to the total concentration

Table 2 Total wastewater concentrations Table 3 Contribution of wastewater stream

Total COD kg/m3 0.675 Loading contribution of: Municipal Industrial

Total N kg/m3 0.053 Total COD 73.5 % 26.5 %

Total P kg/m3 0.007 Total N 86.2 % 13.8 %

Total P 95.9 % 4.1 %

Analysis of the industrial wastewater reveals that the COD consists of alcohols and volatile fatty

acids. This makes the wastewater suitable for treatment in high rate anaerobic reactor. You decide to

investigate the possibility of a UASB reactor.

b) Select and appropriate volumetric loading rate and calculate the expected size of the UASB

As we will use the UASB reactor only for the industrial wastewater stream we choose the VLR from

Table T10-11 from Metcalf & Eddy using the following steps:

- COD = 3750 g/l is in the 2000-6000 g/l range

- Fraction as particulate COD = 0.1-0.3 because we have alcohols and VFAs in the wastewater

- Choose granular sludge with little TSS removal we do not have many particles in the WW

- The VLR should be in the range of 12-18 kg COD/m3*day

- Choose VLR=17 kg COD/m3*day [1]

By choosing a VLR we can now calculate the hydraulic retention time HRT and than the reactor

volume - VR

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c) What will be the dimensions of the anaerobic reactor? Assume and additional water column

of 2 meters above the sludge bed.

As we have COD nearly 100% soluble we choose an up-flow velocity of 1.3 m/s. With this, we can

calculate the diameter and the height of the reactor.

Adding the 2 m water column on top of the height needed for the sludge blanket we get:

d) The expected COD conversion is 95%. Calculate the required amount of nutrients for sludge

growth. Assume an overall yield of 0.05 g biomass-COD/g COD converted. Are there

sufficient nutrients present in the industrial wastewater?

Having a 95% conversion of COD means:

0.95 * 3.75 kg COD/m3 = 3.5625 kg COD/m3 consumed by the microorganisms

With a yield of 0.05 we get:

0.05 * 3.5625 kg COD/m3 = 0.1781 kg TSS/m3 biomass production

For the ratio of 1 g COD / 1g TSS --> 0.1781 kg COD/m3 excess sludge production

Using two empirical relations we can calculate the needed amount of nutrients N & P for the

biomass to grow.

Comparing these values with the influent concentrations we get:

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e) What will be the expected biogas production in m3 CH4/day?

First we need to find how much COD is available for methane production. From the available 95% of

the COD we subtract the amount need by the bacteria.

As we know the flow of the wastewater we can calculate the amount of methane that can be

generated every day:

f) What will be the expected effluent composition (COD and nutrients) from the anaerobic

reactor? (neglect biomass in the effluent)

The effluent concentrations are shown in table 4.

Table 4 UASB effluent characteristics

Calculation Result

COD kg/m3 3.75 3.5652 0.1875

Nitrogen kg/m3 0.154 0.0151 0.1389

Phosphorus kg/m3 0.006 0.0038 0.0022

g) Is separate treatment financially attractive when looking only at the cost of building an

anaerobic reactor (assume 500 eur/m3 reactor) and the potential revenue from electricity

(assume an income of 0.05 eur/kWh) generated from the biogas in a Combined Heat and

Power (CHP) unit? Which other costs/incomes should be included for a detailed economic

evaluation?

First we need to calculate the cost of the reactor:

Knowing the density of methane, we can calculate the mass of methane generated per day and than

calculate the energy output per day. The caloric heat of methane is 55,5 MJ/kg [2]

Converting the MJ into kWh we get:

For heat engines the efficiency of converting fuel into electrical energy is around 25-38% [3]. Using a

=35% efficiency we get the amount of electrical energy that we can get per day:

The value of the electricity is:

The payback time can be easily calculated:

A payback time of 1.6 years is reasonably good and we would recommend the investment into

building a new reactor in the existing plant.

There are some other costs associated with the UASB reactor:

- Maintenance of the turbines

- Pumping costs as you need to pump as high as 9 m

- Salary for the extra personnel

- Sludge disposal can be coupled with the existing sludge disposal system.

Some possible other income sources that can be identified are:

- Selling the heat to nearby houses as hot water

- Extraction of polymers from the intermediate steps of the anaerobic treatment, before

methanogenesis still under research.

h) The effluent of the anaerobic reactor will be discharged to the (aerobic) wastewater

treatment plant. Calculate the composition of the new combined waste stream (domestic +

UASB effluent). What would be the main concern for the treatment plant?

In order to calculate the new mix concentration (UASB effluent + municipal) we follow the same

procedure as we use for calculating the mix of industrial & municipal wastewater. In this case, the

UASB effluent concentrations are smaller than the ones of the industrial wastewater, influencing the

aerobic treatment. Table 5 contains the new influent values and the ones for the previous situation.

Table 5 Influent concentration overview with the new system

Unit UASB + municipal Industrial + municipal

COD kg COD/m3 0.5051 0.675

Total N kg COD/m3 0.0523 0.053

Total P kg COD/m3 0.0068 0.007

The only concentration that presents a high decrease is the one of COD. The 25% decrease translates

into a lower loading rate for the aerobic treatment. The concern for the existing wastewater

treatment plant will be that now it has to work than at under-capacity which will mean lower sludge

retention time and cleaner effluent.

References:

1. Metcalf & Eddy, Wastewater Engineering,Treatment and Reuse - International Edition. Forth Edition ed, ed. McGrawHill. 2004.

2. WolframAlpha.com. Available from: http://wolframalpha.com. 3. US Environmental Protection Agency, N.N.-D., Impact of Combined Heat and Power on

Energy Use and Carbon Emissions in the Dry Mill Ethanol Process. 2007.