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

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.

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)

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).

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

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

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).