28 June 2023

High-Rate Algal Ponds (HRAP) for domestic wastewater treatment

Author/Compiled by
Enrica Uggetti (Universitat Politècnica de Catalunya-BarcelonaTech)
Antonio Ortiz (Universitat Politècnica de Catalunya-Barcelon

Executive Summary

High-Rate Algal Ponds (HRAP) are cost-effective systems designed to enhance wastewater treatment and microalgal biomass production. In this treatment system microalgae assimilate nutrients from wastewater and produce the oxygen needed by bacteria to oxidize the organic matter. The symbiosis between bacteria and microalgae leads to the production of: 1) clean water suitable for reuse, and 2) algal biomass that potentially can be converted in bioproducts such as biofertilizers, biostimulants, biogas, pigments, bioplastics, etc.

HRAPs are suitable for the treatment of urban and industrial wastewater, digester effluent, piggery, and dairy farm wastewater, etc.



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Groundwater sources (borehole, spring)
Raw domestic wastewater (blackwater) – secondary or tertiary treated wastewater
Degreased, screened sewage
Industrial wastewater



Secondary treated effluent
Tertiary treated effluent


Removal of...

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Total suspended solids (TSS)
Total dissolved solids (TDS)
Organic compounds / COD / BOD5 / TOC
% Total Solids (for sludge)


Design considerations

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High-Rate Algal Ponds (HRAP) systems consist of two main parts: a shallow raceway with paddlewheels where wastewater treatment is carried out by microalgae and bacteria, and a harvesting unit to separate the biomass from the treated water, as illustrated in Fig. 1 below.

In HRAPs, pre-treated wastewater is gently moved by paddlewheels. Microalgae use sunlight to grow, remove the nutrients from the wastewater, and produce oxygen through photosynthesis. The oxygen is used by aerobic bacteria to oxidize the dissolved organic matter (PASSOS et al. 2017).

Organic loading rate, depth, hydraulic retention time and horizontal mixing velocity are the main operational control variables for HRAPs. Depending on the climate, the maximum organic loading rate of HRAPs is between 100- and 150-kilogram Biological Oxygen Demand per hectare per day (kg BOD5/ha·d). HRAP depth depends on wastewater clarity. Typical values are 0.2 - 0.6 metres (m). In temperate climates, Hydraulic Retention Time (HRT) varies seasonally (3 - 4 days in summer and 7 - 9 days in winter). Paddlewheel mixing (typically 0.15 - 0.30 metres per second, m/s) causes turbulent eddies that provide a vertical mixing component within the pond so that microalgal cells are intermittently exposed to sunlight. The annual biomass productivity of wastewater treatment HRAPs at moderate latitudes is typically 30 tons per hectare per year (t/ha·y) (CRAGGS et al. 2014)..

The biomass is collected through the harvesting units which consist of a gravity settler or a Dissolver Air Flotation (DAF) system.

Example of HRAP

Figure 1. Example of HRAPs design developped in the framework of the PAVITR project. The HRAPs are built in Aligarh (India). Source: E. UGGETTI (2021)



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This system is based on the microalgae-bacteria synergy, and it is suitable for both nutrients and organic matter removal from different types of wastewaters (urban, industrial, etc.). Pre-treatment with a gravity settler is normally needed to remove part of the solids.

Due to its quite high land requirement (3 - 5 square metres per person equivalent, m2/PE), it is mainly suitable for small communities (2,000 PE), however it could be used for communities of up to 10,000 PE.

HRAP are biological systems based on photosynthetic microorganisms, thus they are working properly in warm and sunny climate. Unfavourable climatic conditions can strongly compromise the system’s performances in terms of wastewater treatment and biomass production.

HRAP systems are characterized by low implementation, operation, and maintenance costs. HRAPs can be built with materials that are normally easily available locally, such as bricks, concrete, steel, or polyvinyl chloride (PVC) (no transport needed). Also, the energy demand of HRAPs (0.02 kilowatt-hours per cubic metre, kWh/m3) is considerably lower than of conventional wastewater treatment technologies such as activated sludge (0.20-0.4 kWh/m3). Even though HRAPs do not require highly specialized operation and maintenance (O&M) staff, they require trained personal (see O&M tasks below).


Operation and Maintenance

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The following monitoring probes are required for proper operation of HRAPs:

  • pH
  • Temperature
  • Dissolved oxygen
  • Water lever


The main operation and maintenance activties are:

  • Checking and maintenance of the probes (weekly cleaning and calibration)
  • Checking of inlet and outlet pumps, as well as proper paddlewheel operation
  • Measuring the turbidity of the settled effluent

Basic knowledge of electronics is required for theses activities.


Main consumables:

  • Monitoring probes
  • Coagulant (e.g. polyaluminium chloride or natural ones) for settler
  • Laboratory reagents to clean the sensors


Experiences in India

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As far as we know, by the time this factsheet was compiled (2023), HRAPs were not yet used in India. 


Experiences Globally

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Experiences with the application of HRAPs at large scale are still very limited. Some systems are built in California and Australia (PARK & CRAGGS 2010). Nowadays in Spain the company FCC Aqualia S.A. is running some large-scale facilities in the south of Spain. However, literature around their experiences is unfortunately only available in Spanish language (DE GODOS et al. 2013).


Library References

Proyecto All-gas Cultivo de microalgas con producción de biocombustibles y eliminación de nutrientes

DE GODOS, I., ZOUHAIR, A., FERIA, M., LARA, E., SANTIAGO, J., ROGALLA, F. , FERNÁNDEZ, M. y DE LA RUBIA, M. (2013): Proyecto All-gas Cultivo de microalgas con producción de biocombustibles y eliminación de nutrientes. In: RETEMA Especial Bioenergía: Volume 169 , pp. 56-63. España: Revista Técnica de Medio Ambiente (RETEMA). URL [Accessed: 22.07.2021] PDF
Further Readings

What are algae?

This paper aims to explain 'what are algae' and how to answer the most relevant questions to different players interested in the field, including: academia, industry, trade organizations, consumers, business investors, local and national authorities, international organizations and any other interested party or stakeholder. This position paper represents the position of EABA as the Algae Biomass sector Association from a European perspective and summarizes information from science, technology and business dealing with 'algae' biomass, biotechnology and bioproduct.

EABA (2021): What are algae?. In: #1 Position Paper - Version 2.0. Firenze (Italy): European Algae Biomass Association (EABA): URL [Accessed: 09.07.2021] PDF

Comprehensive Evaluation of High-Rate Algal Ponds: Wastewater Treatmentand Biomass Production

In this book chapter, a state-of-the-art review of literature is presented to understand the design, governing process of HRAPs system, and role of different environmental/operational parameters on its performance. Effective implementation of HRAPs system will lead to the production of value-added products like potash in the near future. This chapter may help to frame research work related to improvement/implementation of HRAPs system for effective wastewater treatment and biomass production for fertilizers, feeds, and biofuels.

RANJAN, S., GUPTA, P.K. and GUPTA, S.K. (2019): Comprehensive Evaluation of High-Rate Algal Ponds: Wastewater Treatmentand Biomass Production. In: GUPTA, S.K and BUX, F. (eds.) (2019): Application of Microalgae in Wastewater Treatment. Volume 2: Biorefinery Approaches of Wastewater Treatment. 531-548. URL [Accessed: 09.07.2021] PDF

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