synergies of wastewater treatment and microalgae cultivation in austria

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Synergies of wastewater treatment and microalgae cultivation in

Austria

Andrea Sonnleitnera,*, Dina Bacovskya, Günther Bochmannb, Bernhard Drosgb, Michael

Schagerlc

a BIOENERGY 2020+ GmbH, Gewerbepark Haag 3, 3250 Wieselburg-Land, Austria; [email protected], [email protected]

b University of Natural Resources and Life Sciences, Department for Agrobiotechnology, IFA-Tulln, Institute for Environmental Biotechnology, Konrad Lorenz Str. 20, 3430 Tulln, Austria; [email protected], [email protected]

c University of Vienna, Faculty of Life Sciences, Department of Limnology, Team Phycology, Althanstrasse 14, 1090 Vienna, Austria; [email protected]

* Corresponding author: Tel.: +43 7416 52238-37, fax: +43 7416 52238-99, E-mail address: [email protected]

Abstract

Current international research results identify microalgae as a new and promising feedstock for the

global energy supply chain. A novel concept to reduce costs and cover the need of water and nutrients

is the combination of wastewater treatment and microalgae cultivation. In Austria in particular brewery

and dairy effluents as well as municipal wastewater would be suitable for algae cultivation. Cultivation

systems practical for the use of wastewater are High Rate Algal Ponds (open system, suspended

culture), Algal Turf Scrubbers (open system, immobilized culture) and Photobioreactors (closed

systems, suspended culture). The cultivation of microalgae in general and the special case of

wastewater as nutrient source face a variety of challenges either concerning the accumulation of

microalgal cells in wastewater (upstream process) or their removal and processing (downstream

process). Taking a look at the whole production chain shows that for effluents of breweries, dairies

and smale-scale municipal wastewater no feasible concept for the combination of microalgae

cultivation and wastewater treatment can be designed. A promising production concept for large-scale

municipal wastewater treatment plants are HRAPs or biofilm production in ATS systems for energetic

and material pathways. Various R&D challenges are to overcome to lead to an optimization and

further development of technologies for combined wastewater treatment and microalgae cultivation in

Austria.

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Synergies of wastewater treatment and microalgae cultivation in Austria of 10

Background and objectives

Current international research results identify microalgae as a new and promising feedstock for the

global energy supply chain (U.S. DOE, 2010, Darzins et al.,2010, Sikes et al., 2010). According to the

current state of the art the utilization of algal biomass for the production of fuel, energy and heat is not

competitive within the global and national energy market (Hingsamer et al., 2012). Challenges along

the value chain are the supply of water and nutrients for cultivation, the energy consumption during

cultivation, harvesting and processing of biomass and investment and operating costs.

One low-cost option to cover the need of water and nutrients is the combination of microalgae

cultivation and wastewater treatment. The biocoenosis of phototrophic microalgae and bacteria in

wastewater is depicted in Figure 1. During photosynthesis, microalgae provide dissolved oxygen

which is used by bacteria to break down and oxidize wastes. This leads to liberation of CO2,

phosphorus, nitrogen and other nutrients used by algae. This interaction of bacteria and algae results

in purification of effluents and an uptake and storage of nutrients and CO2 in organic biomass.

Figure 1: Basic principles of operation in an integrated algae culture (U.S. DOE, 2010)

The major advantages and disadvantages of combined microalgae cultivation and wastewater

treatment are listed in Table 1.


Table 1: Evaluation of combined algae cultivation and wastewater treatment (Brennan et al., 2010; Harun et al.,

2010; Lundquist, 2008; U.S. DOE, 2010)

Advantages Disadvantages

Replacement of chemical remediation techniques High land requirements for open pond systems and high

investment costs for PBR systems

Increasing performance of degradation and purification Longer detention times (at least several days) than

conventional wastewater treatment (< 0.5 days)

Improved CO2 balance and reduced GHG emissions Contaminated wastewater requires appropriate pretreatment

to remove sediment and deactivate the wastewater

Lower investment, operating and maintenance costs than

conventional wastewater treatment

Algae containing heavy metals are difficult to convert or

dispose of

Wastewater revenue offsets algal production cost Fail to meet suspended solid limits (< 45 mg/l)

Minimization of fresh water use for algae cultivation Utilization of biomass for food or feed is legally not feasible

Algae lower the energy demand for oxygen supply Lower productivity compared to conventional systems

For a potential enhancement of biofuel production from microalgae the combination of algal biomass

production and wastewater treatment needs to be improved and optimized. Suitable wastewater

streams have to be identified and investigated, promising cultivation systems have to be found for the

respective wastewater and promising production concepts for the climatic areas need to be

developed.

Wastewater streams suitable for microalgae cultivation

Microalgae cell growth is influenced by light, temperature, water, gas exchange, pH value, mixing and

a variety of nutrients like phosphorus or nitrogen.

In Austria different types of water and effluents rich in nutrients are available:

■ Municipal wastewater: Municipal wastewater plants in Austria can be divided into large scale

plants (local wastewater associations) and small scale plants (< 1000 PE (population

equivalent)). These effluents contain sufficient amounts of nutrients and are available over

the whole of Austria in large quantities.

■ Industrial wastewater: Industrial effluents are any large scale wastewater from a variety of

industry branches. For Austria these are listed in the Wastewater Emission Ordinances

(Abwasseremissionsverordnungen AEVs) as supplement to the Water Act (WRG 1959 as

applicable). Some of these wastewater streams can be ruled out in advance like effluents of

the medical sector (hazardous), textile industry (chemicals in effluent, low nutrient content),

chemical, metallurgical and mineral processing industry (insufficient nutrient content). Other

effluents would contain enough nutrients for microalgae cultivation but are not suited for

other reasons like e.g. raw effluent from slaughterhouses, which are hazardous and contain

pathogens. Suitable wastewater streams are from food processing industry, like effluents

from breweries or dairies.

■ Agricultural wastewater: Agricultural wastes can be either of organic nature from animal

facilities (swine, dairy operations, aquaculture) or agricultural drainage with low organic but


high nutrient content (U.S. DOE, 2010). The major emphasis on this wastewater treatment

lies in the removal of nutrients and the reduction of biological oxygen demand BOD

(Benemann, 2003). Liquid manure or slurry has to be strongly diluted with water for algae

cultivation, because of its colour and high ammonia content. Since common procedures for

the use of slurry (fertilizer for fields, feedstock in biogas plant) exist and large quantities of

water are needed for dilution, the utilization of slurry seems to be inappropriate for industrial

microalgae cultivation.

■ Eutrophic bodies of water: In Austria hardly any eutrophic body of water can be found (H2O,

2013; WISA, 2013). Even the rivers with water quality of the category II-III have low nutrient

contents and therefore are not suited for active algae cultivation.

Within the project SAM (synergies of wastewater treatment and microalgae cultivation) four potentially

suitable wastewater streams for industrial microalgae cultivation in Austria were identified and

investigated (Table 2):

Table 2: Suitable wastewater streams in Austria

Type of wastewater Available amount in Austria

Brewery effluent 2.7 – 4.5 Mio. m3/a

Dairy effluent 3.6 Mio. m3/a

Municipal wastewater large scale 450 Mio. m

3/a

Municipal wastewater small scale

Cultivation systems

For the application of algae cultivation different systems are existing, which can be divided into 2

groups: open and closed systems. Open systems are Open Ponds, High Rate Algal Ponds (HRAP),

Algal Turf Scrubbers (ATS) and other non-closed cultivation systems. Closed systems comprise any

type of Photobioreactor (PBR, tubular, plates, helical, plastic bags, etc.) which enable autotrophic

cultivation and Fermenters in which heterotrophic or mixotrophic cultivation takes place. The latter are

not further investigated, since energy is not gained via photosynthesis.

Another way to divide the cultivation systems is into systems with suspended cultures and systems

with immobilized cultures (Christenson and Sims, 2011). Suspended cultures are found in HRAPs and

PBRs. Systems with immobilized cultures are matrix-immobilized or biofilm systems (e.g. ATS).

Matrix-immobilized systems are not suitable for low-tech application because of high costs.

Cultivation systems practical for the use of wastewater are HRAPs (open system, suspended culture),

ATS (open system, immobilized culture) and PBRs (closed system, suspended culture). Each of these

systems offers various advantages and disadvantages at defined cultivation parameters. These are

listed in Table 3 together with pictures and schemes of the cultivation systems.


Table 3: Pictures, graphs, advantages and disadvantages of cultivation systems with wastewater

HRAP

source: AlgaeParc Wageningen

Chisti 2007

Advantages: Disadvantages:

Low investment and operating costs difficulties with process control

a lot of practical experience low algae cell concentration

moderate oxygen levels contaminations

easy scale-up low illuminated surface

cooling due to evaporation high evaporation rates

ATS

Mulbry et al. 2008

Craggs et al., 1997


low investment and operating costs difficulties with regulation of process

high illuminated surface low volume

easy harvesting high land requirements

easy scale-up contaminations

easy gas exchange with atmosphere

no need of external CO2 insertion

low oxygen levels

PBR

source: AlgaeParc Wageningen

Chisti 2007


good process control high investment and operating costs

high illuminated surface Adherence of algae cells on surface

high productivities high oxygen levels

high cell densities difficult scale-up

low evaporation losses regulation of temperature necessary

(overheating)


The cultivation of microalgae offers a variety of challenges. These challenges either concern the

accumulation of microalgal cells (upstream process) or their removal and processing (downstream

process). For successful algae cultivation these obstacles have to be eliminated, reduced or avoided.

The challenges of microalgae cultivation are diverse and have to be considered separately for each

application:

■ Sufficient supply of nutrients

■ Gastransfer and exchange

■ Supply of photosynthetic active radiation (PAR)

■ Avoidance of contamination

■ Process control

■ Choice of location

■ Climatic conditions in Austria

■ Land requirements near water and nutrient sources

■ Scale-up and large-scale application

■ Harvesting

■ Water removal

■ Further processing

Using wastewater for cultivation of microalgae leads to further problems and restrictions in the use and

choice of cultivation systems. Possible problems are that an already existing mix of microorganisms

handicaps a pure culture, low or no efficiency of purification in winter season without external heating

and lighting, fluctuating wastewater qualities influence continuous operation, inhibitors in wastewater

decrease microalgal growth.

Development of production concepts

The combination of microalgae cultivation and wastewater treatment is influenced by several factors

along the value chain. Production concepts have to consider these factors and unite them into an

entire system. Besides suitable wastewater streams, microalgal species, cultivation and harvesting

systems also exploitation schemes and economic aspects need to be considered.

The identified suitable wastewater streams (brewery, dairy, municipal (small-scale, large-scale)) were

technologically assessed:

■ Wastewater from large-scale municipal wastewater treatment plants show high potential for

combination with microalgae cultivation. This production concepts could be applied all over

Austria since the availability of municipal effluents comprise several hundred million

cubicmeters per year in over 635 plants (PE > 2.000, Überreiter et al., 2012) scattered all

over the country. The wastewater contains sufficient nutrients, is available in large quantities

in Austria, sufficient free areas around existing plants are available, and additional process

flows like waste heat and flue gas from the combined heat and power (CHP) plant can be

used. The microalgae cultivation serves as preliminary purification step and biomass


production. The produced biomass can be used as feedstock for products or energy and

generates additional profit. Disadvantageous are the increased amount of sludge produced

and the high land requirements if the whole wastewater stream should be purified. The

production concept for microalgal biomass in municipal wastewater treatment plants is

shown in Fehler! Verweisquelle konnte nicht gefunden werden.. Possible production

concepts for large-scale municipal wastewater treatment plants are HRAPs or biofilm

production in ATS systems for energetic and material pathways.

Figure 2: Production concept of combined municipal wastewater treatment and cultivation of microalgae

■ In municipal small-scale wastewater treatment plants no technological or economical benefit

can be derived in combination with microalgae cultivation. The efficiency of purification with

algae in this system is very low (single not loop system) and the harvesting and further

utilization of biomass is practically not feasible because of the decentrality of the systems.

The combination of already existing plants seems to be infeasible, for these systems novel

purification technologies based on microalgae have to be developed.

■ In dairy processes the use of microalgae is economically not feasible, since no high-value

products can be obtained using wastewater as nutrient source. One option is to use liquid

leftovers like whey which is not declared as wastewater. Biomass produced out of whey can

be processed in a biorefinery concept into high-value products (feed, fertilizer, cosmetics,

bioplastics, chemicals, etc.) and energy can be gained from the residues.


■ Brewery effluents contain a relatively low content of nutrients. Only a low additional benefit

can be generated because for high-value products using wastewater is not legally feasible

and no liquid leftovers are available which are not declared as waste.

Need for research and development

The synergies of microalgae cultivation and wastewater treatment face research and development

challenges along the entire production chain (Figure 3). The microalgae cultivation in Austria, the

special case of wastewater as substrate, the harvesting and utilization have to be investigated further

for the particular production concept and adapted to the given circumstances. Possible production

concepts were already developed but the feasibility in real plants has to be proved. The overall

assessment along the value chain has to be done regarding economic aspects (economic efficiency)

and ecologic aspects (life cycle assessment). Based on these results the production concepts and

their feasibility can be assessed.

Figure 3: Need of research and development along the entire process chain of combined wastewater treatment

and microalgae cultivation

Further need of research and development for combined microalgae cultivation and wastewater

treatment is given on different levels. Besides lab experiments and studies especially long-term

experiment in pilot plants are of essential importance.

■ Fundamental questions can be answered in lab experiments, like definition of inhibitors in

wastewater, adequate dilution rates, species of microalgae, composition of biofilm and

potential of material and energetic utilization pathways.

■ Additionally theoretical studies and assessments regarding economic efficiency, ecological

balancing, greenhouse gas emissions, choice of location and declaration of biomass (waste

or reusable material) have to be conducted.

■ The major part of the research can be covered with long-term experiments in pilot plants.

Within these experiments well founded and sound data are generated for further

assessments. In pilot plants the influence of the climatic conditions in Austria and of the


fluctuating wastewater quality on the efficiency of purification can be determined. Cultivation

systems can be adapted to the outer circumstances (type of wastewater, species of

microalgae, favourable product, harvesting system) and optimised with regard to energy

requirements, costs and technology. The potential of various energetic utilization pathways

has to be investigated further (e.g. biogas, hydrothermal treatment).

Perspectives and outlook

Taking a look at the whole production chain shows that possible production concepts for large-scale

municipal wastewater treatment plants are HRAPs or biofilm production in ATS systems for energetic

and material pathways. This production concept is promising – however various R&D challenges are

to overcome. For effluents of breweries, dairies and small-scale municipal wastewater no feasible

concepts for the combination of microalgae cultivation and wastewater treatment can be designed.

The results of such further research activities will lead to an optimization and further development of

technologies for combined wastewater treatment and microalgae cultivation. Based on reliable data on

material and energy flows in pilot plants the economic and ecologic assessment can be carried out. All

these activities determine the course for the implementation of technologies for industrial and

municipal wastewater treatment combined with algal biomass production.

Acknowledgements

The work was carried out within the project “SAM – Synergien von Abwasserreinigung und

Mikroalgenkultivierung”. Financial support comes from the Austrian Research Promotion Agency

(FFG) in the framework of the Research, Technology and Innovation (RTI) initiative “Intelligent

Production” on behalf of the Austrian Federal Ministry for Transport, Innovation and Technology

(BMVIT) and from Güssing Renewable Energies GmbH.


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synergies of wastewater treatment and microalgae cultivation in austria

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