Concept (1) (Autosaved)

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This report addresses major considerations for designing commercial algae based wastewater

treatment technology (AWTT) to treat industrial wastewater containing high amounts of nitrates,

phosphates and heavy metals. AWTTs design mainly focuses on providing a technology that is

cost effective, requires low maintenance, and has minimal operator requirements. AWTT design

is divided into two sections: the first section being devoted to algae growth under heterotrophic

conditions and other for treating industrial wastewater in open race way pond. Both these

sections are discussed in more detail below.

In this design open raceway pond was used for treating wastewater, a

technology already used in commercial microalgae production plants and some pilot scale

biofuels projects. The design of this report differs from existing commercial designs in

cultivating the algae heterotrophic ally in a continuous stirred tank reactor prior to treating

wastewater. It was experimentally proven in our lab scale system that heterotrophically grown

algae can efficiently clean wastewater containing nitrates, phosphates and heavy metals (Cr, Cd).

A correlation was developed from the experimental studies for metal uptake of chlorellavulgaris

as a function of biomass concentration, initial metal concentration, time, pH, temperature and

light intensity. Using this correlation, for a given contaminant (nitrates, phosphates, metals)

concentration in the raceway pond, contact time and initial biomass needed to treat wastewater

can be found. Cost and time to clean wastewater can be reduced considerably by growing algae

heterotrophically initially as lag phase of algae can be avoided in the race way pond. Also,

known concentration of algae can be introduced directly into the pond depending on the

contaminant present in the wastewater.


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Technology assumptions

This report assumes that a piping network or some other collection system (e.g., tank trucks) is in

place that carries wastewater from individual sources to the AWTT plant. It also assumes that

carbon dioxide from flue gas was separated and stored near treatment facility to feed the algae.

Paddlewheels, 2-hp, 1-phase, 230V/60 Hz approximately 5m wide were used to mix the pond.

Paddle wheel rotates providing a current of 25cm/s around the pond to ensure that all of algae

receive necessary amounts of carbon dioxide and solar radiation. Volume of wastewater treated

in AWTT design was assumed to be 200,000 liters. To meet this volume, a cultivation reactor of

volume 25 m

3

and a race way pond of 1000 m

2

area were designed. Race way pond is 77m in

length, 14 m wide, and 20 cm in depth corresponding to a volume of 200,000 liters. In CSTR,

reactor tank is assumed to be cylindrical with tank diameter equal to height of the reactor.

Glucose concentration of 3 g/L was used as an organic carbon source for heterotrophic cell

growth. A large centrifuge which rotates at 20000 r.p.m was installed at the harvest station of

race way pond.

Site location and climatic conditions:

Tropical or semi-tropical areas are the most practical locations for treating wastewater. The

location chosen for construction has to consider the following:

(a)Evaporation: Evaporation is a major problem in dry tropical areas. A high evaporation

rate increases salt concentration in wastewater and pumping cost due to water loss.

(b)Humidity: With high relative humidity and no winds the water may heat up (even to 40

oC). With low humidity, high rates of evaporation occur that can have a cooling effect on

the medium. The best humidity is at an average humidity of 60%.


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(c)Water: A location must be chosen where there is constant and abundant supply of

wastewater (may be location close to wastewater treatment plant).

Design theory:

A schematic of the AWTT bioreactor set-up that is being designed for treating wastewater is

shown in Figure 1. It mainly consists of the heterotrophic growth chamber (CSTR) of volume 25

m3 and an open race way pond of area 1000 m2.

(a) Heterotrophic growth chamber

The components that make up the heterotrophic growth chamber include the bioreactor,

compressor and mixer. Reactor was jacketed to maintain constant temperature throughout the

process. All sensors (temperature, level, pH, pressure and dissolved oxygen) are installed in the

bioreactor. An impeller driven by a motor was installed inside the reactor to mix the feed and to

keep algae solution homogenous. Compressed air was passed through the air filter and

introduced at the bottom of the bioreactor through a sparger. Bioreactor was equipped with air

outflow protected from contamination.

To design a cost effective bioreactor, the starting point of the optimization design is a chosen

bioreactor size, with specific algae of known biomass concentration (0.1 g/L).A flowchart of the

entire design optimization strategy used is given in Figure 2. The details of each step, with

accompanying equations, constraints, and references follow. The main focus of the strategy is to

minimize the cost of the CSTR. The total cost includes both the capitalcost of the equipment and

the operating cost.


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Flow chart 1: Bioreactor design is based on required reactor volume. A working volume of 25

m3was assumed initially to design the bioreactor. Working volume would be 2/3rd of tank height

to achieve uniform mixing throughout the reactor.

Flow chart 2: The impeller selected was standard six blade radial flow design used most often in

bioreactors as shown in Figure 2.It creates a turbulent flow pattern that improves mixing and

uniformity.

2. Select impeller type

3. Specify minimum stirring rate

1. Design bioreactor of volume 25 m

6. Calculate total cost

5. Calculate power for aerated

bioreactor

4. Select gas rate and calculate

minimum superficial velocity

11. Calculate oxygen

concentration in

exhaust

10. Calculate cost of compresso

and mixer

9. Calculate total external power

8. Calculate compressor power

7. Calculate unaerated power

12. Increase mixing

speed/ aeration rate


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Figure 3 : Impeller ( DI/w = 5); DI = 0.40 d.

Flow chart 3: The stirring rate is optimized, with a minimum value (for stirring the liquid media

alone) determined from (Oldshue, 1983):

Nmin= 1.22 + 1.25 (d/ DI) (g/L)0.25(1/ DI)

Flowchart 7: Calculation of the power required for the unaerated reactor is based on a turbine

power correlation for the chosen impeller (Perry and Green, 1997).

For impeller style 1 (valid for transitional and turbulent conditions 66 NRE 50,000; for NRE>

50,000, NP= 5):

NP= 2.5737NRE0.0614,

Where NP = Po / ( LN3DI

5)

NRE= (NDI2) /

Flow chart 4:The aeration rate is optimized to take out the dissolved oxygen from the reactor, an

initial value was chosen as 0.1 vvm (volume air / volume liquid / min) (Elizier, 1987) is assumed

so Q = (0.1/60) VWm3/s.

Superficial velocity = Q / (d2/4),

Flow chart 8:The required compressor power needed for aeration is determined by the equation

Wc= 2*R*298 mair 3( 1/ ) - 1

MW air( 1)


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Flow chart 5 :Power needed for stirring the aerated reactor ,

Pa= 0.8 Po

(1 -G) NA0.38NWE

0.25

Where the aeration number is

NA= Q/NDl3

And the Weber number is

NWE= N2 Dl

3 L /

The gas holdup for a well-mixed bioreactor like this CSTB (Hassan and Robinson, 1977) is

determined from the standard equation for pure water as the culture liquid.

G= 0.113 QN2/

0.57

The costs of the compressor and the mixer are capital costs, but are still to be minimized with the

operating costs; as they are affected by the determination of the aeration rate and stirring speed.

Only one type of compressor and one type of mixer is assumed to be available for the purposes

of this CSTB design and optimization. The capital cost correlations are based on the industrial

cost data of Ulrich and Vasudevan (2004).

CBMC= 2.5 932.6 Wc0.9373

The correlation is valid for $35,000 < CBMC < $ 3,800,000, and 47 kW < WC < 7000 kW.


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The mixer cost correlation is given below for a stainless steel mechanical seal agitator. It is

suitable for use with nearly all culture fluids and tolerates pressures up to 8100 kPa. It includes

motor, speed reducer, and impeller cost.

CBMM= 2.5 847.65 PG/1000 + 8423.6

The correlation is valid for $8,100 < CBMM< $ 183,000, and 1.3 kW < (PG/1000) < 206 kW.

The total cost involved for installation Ct= CBMC + CBMM

The total external power required by the CSTB is the sum of power used by compressor and

mixer.

PEXT= c Wc + M (Pa/1000)

Flow chart 6:The final consideration in the program is the total cost of the CSTB as designed.

This is an optimization condition in the program and includes both capital and operating costs.

Operating costs are for utilities only and are determined for 5 years of operation or 1000 days.

The bioreactor capital cost is

CF = 40,000 (Vtank/0.5)0.6

(b) Open race way pond

The 'raceway pond' is a large open water raceway track where algae and wastewater are pumped

around by a motorized paddle. The depth of race way pond used in this design is 20 cm

corresponding to a volume of 200,000 liters. The pond is 14x77m, plastic lined with concreteblock walls and a central wooden divider.Monitoring equipments were installed within the pondfor monitoring conductivity, pH, algal density, temperature and contaminant concentration. Pond

was constantly mixed with paddle wheel to maintain cells in suspension, to provide good contact


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between algae and wastewater and to prevent thermal stratification. Ample amount of carbon

dioxide was infused into the pond all the time. The CO2is injected into the algae pond in the

form of bubbles and the bubblers are spaced around the pond so that the CO2is evenly dispersed

throughout the pond. A centrifuge installed at the harvesting station which rotates at several

thousand rpm was used to separate algae and contaminant free wastewater. The major

disadvantage in using centrifuge is the high cost associated with operating the centrifuge.

Design principle

This section is a step by step walk through of the Algae based Wastewater Treatment

Technology (AWTT). Initially growth media (BBM) of volume 20 m

3

was fed into the

bioreactor along with algae solution of volume 5 m3having a concentration of 0.1g/L.

Heterotrophic growth media was supplemented with a d-glucose solution at a concentration of

5 g/L. The culture was stirred by continuous bubbling of sterile air.

This section is a step by step walk through of the algae to biodiesel process.


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Contaminant Initial concentrationin the influent (ppm)

Initial drybiomass

concentration(g /L)

pH Temperature(0C)

Residencetime in the

pond (days)

Finalconcentrationin the effluent

(ppm)

Nitrate 187 0.2 6.7 27 7 < 2 ppm

Phosphate 31 0.327 6.7 27 7 20 ppm

Chromium 30 1.45 6.7 27 2 < 2 ppm

Cadmium 150 1.53 6.7 27 1 < 4 ppm


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Bioreactor design is a relatively complex engineering task, which is studied in the discipline

ofbiochemical engineering.Under optimum conditions, the microorganisms or cells are able to perform

their desired function with a 100 percent rate of success.

[citation needed]

The bioreactor's environmentalconditions like gas (i.e., air,oxygen,nitrogen,carbon dioxide)flow rates, temperature,pHand dissolved

oxygen levels, andagitationspeed/circulation rate need to be closely monitored and controlled.[citation needed]

Most industrial bioreactor manufacturers use vessels,sensorsand acontrol systemnetworked

together.[citation needed]

Foulingcan harm the overall sterility and efficiency of the bioreactor, especially theheat exchangers.To

avoid it, the bioreactor must be easily cleaned and as smooth as possible (hence the round shape).[citation

needed].

Aheat exchangeris needed to maintain the bioprocess at a constant temperature.Biological fermentation

is a major source of heat, therefore in most cases bioreactors need refrigeration.They can be refrigerated

with an external jacket or, for very large vessels, with internal coils.

In an aerobic process, optimal oxygen transfer is perhaps the most difficult task to accomplish.Oxygenis

poorly soluble in watereven less in fermentation broths[citation needed]

and is relatively scarce

inair(20.95%). Oxygen transfer is usually helped by agitation, which is also needed to mix nutrients and

to keep the fermentation homogeneous. There are, however, limits to the speed of agitation, due both to

high power consumption (which is proportional to the cube of the speed of the electric motor) and to the
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damage to organisms caused by excessive tip speed. In practice, bioreactors are often pressurized; this

increases thesolubility of oxygenin water.

The starting point of the optimization design is a chosenbioreactor size, with specific algae of

known biomass concentration (0.1 g/L).A flowchart of the entire design optimization strategy

used is given in Figure 2. The details of each step, with accompanying equations, constraints,

and references follow. The main focus of the strategy is to minimize the cost of the CSTR. The

total cost includes both the capitalcost of the equipment and the operating cost.
http://en.wikipedia.org/wiki/Henry%27s_lawhttp://en.wikipedia.org/wiki/Henry%27s_lawhttp://en.wikipedia.org/wiki/Henry%27s_lawhttp://en.wikipedia.org/wiki/Henry%27s_law


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The microalgae that are known to grow rapidly in presence of organic carbon source in

heterotrophic chamber were used as a starting culture to inoculate open race way pond which is

further used for treating wastewater. It was observed during experimental studies that under

heterotrophic conditions with 3 g L1of glucose, a maximum cell concentration of 10.21 g dry mass

L1was observed for over 7 days of cultivation. The volume of algae grown in heterotrophic

chamber used to inoculate raceway pond depends on the type of the contaminant present in

wastewater. As mentioned in the table xx, different biomass concentrations are required for

treating nitrates, phosphates and heavy metals in wastewater. The operating cost consists of the

power required to aerate and stir the cell culture over a 5 year period.


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Concept (1) (Autosaved)