Rupert J. Craggs

Wastewater treatment high rate algal ponds for biofuel production.

While research and development of algal biofuels are currently receiving much interest and funding, they are still not commercially viable at todays fossil fuel prices. However, a niche opportunity may exist where algae are grown as a by-product of high rate algal ponds (HRAPs) operated for wastewater treatment. In addition to significantly better economics, algal biofuel production from wastewater treatment HRAPs has a much smaller environmental footprint compared to commercial algal production HRAPs which consume freshwater and fertilisers. In this paper the critical parameters that limit algal cultivation, production and harvest are reviewed and practical options that may enhance the net harvestable algal production from wastewater treatment HRAPs including CO(2) addition, species control, control of grazers and parasites and bioflocculation are discussed.

Wastewater treatment and algal production in high rate algal ponds with carbon dioxide addition.

High rate algal ponds (HRAPs) provide improved wastewater treatment over conventional wastewater stabilisation ponds; however, algal production and recovery of wastewater nutrients as algal biomass is limited by the low carbon:nitrogen ratio of wastewater. This paper investigates the influence of CO(2) addition (to augment daytime carbon availability) on wastewater treatment performance and algal production of two pilot-scale HRAPs operated with different hydraulic retention times (4 and 8 days) over a New Zealand Summer (November-March, 07/08). Weekly measurements were made of influent and effluent flow rate and water qualities, algal and bacterial biomass production, and the percentage of algae biomass harvested in gravity settling units. This research shows that the wastewater treatment HRAPs with CO(2) addition achieved a mean algal productivity of 16.7 g/m(2)/d for the HRAP(4d) (4 d HRT, maximum algae productivity of 24.7 g/m(2)/d measured in January 08) and 9.0 g/m(2)/d for the HRAP(8d) (8 d HRT)). Algae biomass produced in the HRAPs was efficiently harvested by simple gravity settling units (mean harvested algal productivity: 11.5 g/m(2)/d for the HRAP(4d) and 7.5 g/m(2)/d for the HRAP(8d) respectively). Higher bacterial composition and the larger size of algal/bacterial flocs of the HRAP(8d) biomass increased harvestability (83%) compared to that of HRAP(4d) biomass (69%).

Algal biofuels from wastewater treatment high rate algal ponds.

This paper examines the potential of algae biofuel production in conjunction with wastewater treatment. Current technology for algal wastewater treatment uses facultative ponds, however, these ponds have low productivity (∼10 tonnes/ha.y), are not amenable to cultivating single algal species, require chemical flocculation or other expensive processes for algal harvest, and do not provide consistent nutrient removal. Shallow, paddlewheel-mixed high rate algal ponds (HRAPs) have much higher productivities (∼30 tonnes/ha.y) and promote bioflocculation settling which may provide low-cost algal harvest. Moreover, HRAP algae are carbon-limited and daytime addition of CO(2) has, under suitable climatic conditions, the potential to double production (to ∼60 tonnes/ha.y), improve bioflocculation algal harvest, and enhance wastewater nutrient removal. Algae biofuels (e.g. biogas, ethanol, biodiesel and crude bio-oil), could be produced from the algae harvested from wastewater HRAPs, The wastewater treatment function would cover the capital and operation costs of algal production, with biofuel and recovered nutrient fertilizer being by-products. Greenhouse gas abatement results from both the production of the biofuels and the savings in energy consumption compared to electromechanical treatment processes. However, to achieve these benefits, further research is required, particularly the large-scale demonstration of wastewater treatment HRAP algal production and harvest.

Recycling algae to improve species control and harvest efficiency from a high rate algal pond.

This paper investigates the influence of recycling gravity harvested algae on species dominance and harvest efficiency in wastewater treatment High Rate Algal Ponds (HRAP). Two identical pilot-scale HRAPs were operated over one year either with (HRAP(r)) or without (HRAP(c)) harvested algal biomass recycling. Algae were harvested from the HRAP effluent in algal settling cones (ASCs) and harvest efficiency was compared to settlability in Imhoff cones five times a week. A microscopic image analysis technique was developed to determine relative algal dominance based on biovolume and was conducted once a month. Recycling of harvested algal biomass back to the HRAP(r) maintained the dominance of a single readily settleable algal species (Pediastrum sp.) at >90% over one year (compared to the control with only 53%). Increased dominance of Pediastrum sp. greatly improved the efficiency of algal harvest (annual average of >85% harvest for the HRAP(r) compared with ∼60% for the control). Imhoff cone experiments demonstrated that algal settleability was influenced by both the dominance of Pediastrum sp. and the species composition of remaining algae. Algal biomass recycling increased the average size of Pediastrum sp. colonies by 13-30% by increasing mean cell residence time. These results indicate that recycling gravity harvested algae could be a simple and effective operational strategy to maintain the dominance of readily settleable algal species, and enhance algal harvest by gravity sedimentation.

Wastewater treatment high rate algal ponds (WWT HRAP) for low-cost biofuel production.

Growing energy demand and water consumption have increased concerns about energy security and efficient wastewater treatment and reuse. Wastewater treatment high rate algal ponds (WWT HRAPs) are a promising technology that could help solve these challenges concurrently where climate is favorable. WWT HRAPs have great potential for biofuel production as a by-product of WWT, since the costs of algal cultivation and harvest for biofuel production are covered by the wastewater treatment function. Generally, 800-1400 GJ/ha/year energy (average biomass energy content: 20 GJ/ton; HRAP biomass productivity: 40-70 tons/ha/year) can be produced in the form of harvestable biomass from WWT HRAP which can be used to provide community-level energy supply. In this paper the benefits of WWT HRAPs are compared with conventional mass algal culture systems. Moreover, parameters to effectively increase algal energy content and overall energy production from WWT HRAP are discussed including selection of appropriate algal biomass biofuel conversion pathways.

Increased pond depth improves algal productivity and nutrient removal in wastewater treatment high rate algal ponds

Depth has been widely recognised as a crucial operational feature of a high rate algal pond (HRAP) as it modifies the amount of light and frequency at which microalgal cells are exposed to optimal light. To date, there has been little focus on the optimisation of microalgal performance in wastewater treatment HRAPs with respect to depth, with advice ranging from as shallow as possible to 100 cm deep. This paper investigates the seasonal performance of microalgae in wastewater treatment HRAPs operated at three different depths (200, 300 and 400 mm). Microalgal performance was measured in terms of biomass production and areal productivity, nutrient removal efficiency and photosynthetic performance. The overall areal productivity significantly increased with increasing depth. Areal productivity ranged from 134 to 200% higher in the 400 mm deep HRAP compared to the 200 mm deep HRAP. Microalgae in the 400 mm deep HRAP were more efficient at NH4-N uptake and were photosynthetically more efficient compared to microalgae in the 200 mm deep HRAP. A higher chlorophyll-a concentration in the 200 mm deep HRAP resulted in a decrease in photosynthetic performance, due to insufficient carbon supply, over the course of the day in summer (as indicated by lower α, Pmax and oxygen production) compared to the 300 and 400 mm deep HRAPs. Based on these results, improved areal productivity and more wastewater can be treated per land area in the 400 mm deep HRAPs compared to 200 mm deep HRAPs without compromising wastewater treatment quality, while lowering capital and operational costs.

Universal temperature model for shallow algal ponds provides improved accuracy.

While temperature is fundamental to the design and optimal operation of shallow algal ponds, there is currently no temperature model universally applicable to these systems. This paper presents a model valid for any opaque water body of uniform temperature profile. This new universal model was tested against 1 year of experimental data collected from a wastewater treatment high rate algal pond. On the basis of 1 year of data collected every 15 min, the average errors of the predicted afternoon peak and predawn minimum were both only 1.3 °C and the average error between these extremes was just 1.2 °C. In order to demonstrate the improvement in accuracy gained, the expressions for heat fluxes used in nine prior temperature models were systematically substituted into the new universal model and evaluated against the experimental data. Errors in the peak and minimum temperatures increased by up to 2.1 and 3.2 °C, respectively, while the error between these extremes increased by up to 2.9 °C. In practical applications, these levels of inaccuracies could lead to an under/overestimation of the algal productivity and the evaporative water loss by approximately 40% and 300%, respectively.

Solar-powered aeration and disinfection, anaerobic co-digestion, biological CO2 scrubbing and biofuel production: the energy and carbon management opportunities of waste stabilisation ponds

Waste stabilisation pond (WSP) technology offers some important advantages and interesting possibilities when viewed in the light of sustainable energy and carbon management. Pond systems stand out as having significant advantages due to simple construction; low (or zero) operating energy requirements; and the potential for bio-energy generation. Conventional WSP requires little or no electrical energy for aerobic treatment as a result of algal photosynthesis. Sunlight enables WSP to disinfect wastewaters very effectively without the need for any chemicals or electricity consumption and their associated CO(2) emissions. The energy and carbon emission savings gained over electromechanical treatment systems are immense. Furthermore, because algal photosynthesis consumes CO(2), WSP can be utilised as CO(2) scrubbers. The environmental and financial benefits of pond technology broaden further when considering the low-cost, energy production opportunities of anaerobic ponds and the potential of algae as a biofuel. As we assess future best practice in wastewater treatment technology, perhaps one of the greatest needs is an improved consideration of the carbon footprint and the implications of future increases in the cost of electricity and the value of biogas.

Wastewater Treatment and Algal Biofuel Production

Promoting algal production in wastewater treatment high rate algal ponds (HRAPs) by CO2 addition enables cost effective near tertiary-level wastewater treatment to be achieved and the harvested algal biomass by-product can be used for biofuel production. Naturally occurring algae thrive on wastewater providing the oxygen for aerobic bacteria to break down the waste to ammonia, phosphate and CO2 which are then assimilated into new algal biomass. Low C:N ratios in wastewater mean that additional CO2 added to HRAP will enable all the wastewater N to be assimilated into algal biomass. CO2 may be easily obtained at the treatment plant as exhaust gas from biogas power generation. CO2 addition to wastewater treatment HRAPs has a further benefit in enhancing bioflocculation of the algal-bacterial biomass to enable low-cost harvest by gravity settling. Although there are several options to convert harvested algal biomass to biofuel, processes that use the entire biomass with little or no dewatering are preferable, and for wastewater treatment plants, anaerobic digestion of the algal biomass along with settled primary sludge is the most easily implemented and economic technology. Since the capital and operation costs of wastewater treatment HRAP are covered by their wastewater treatment role, they provide a cost-effective, even if niche, opportunity for algal biofuel production that could be of great value to the local community. Moreover, GHG abatement and nutrient fertilizer recovery provide additional environmental and financial incentives. Upgrading existing wastewater treatment facultative ponds (used world-over for secondary-level wastewater treatment) to tertiary treatment HRAPs provides an avenue to refine operation and performance issues of these ponds at large(hectare)-scale, for future application to standalone algal biofuel systems.

Advanced pond system for dairy‐farm effluent treatment

Abstract Two‐stage oxidation ponds have traditionally been used for the treatment of dairy‐farm wastewater in New Zealand, but are now considered unsuitable to discharge to waterways. The first full‐scale dairy‐farm advanced pond system (APS), a low‐cost and effective upgrade option for traditional ponds was evaluated over a 2‐year period. The system consisted of an anaerobic pond (AP) (the first pond of traditional oxidation pond systems), a high rate pond (HRP), a pair of algae settling ponds (ASP) and a maturation pond (MP) (which all replace the second pond of traditional systems). APS effluent quality was considerably higher than that of traditional ponds, with respective median effluent concentrations of biological oxygen demand: 43 versus 98 g m−3, total suspended solids: 87 versus 198 g m−3, ammoniacal nitrogen: 39 versus 106 gm−3, total phosphorus: 19 versus 27 g m−3, and Escherichia coli of 918 versus 70 000 MPN/100 ml. APS show great promise for upgrading traditional dairy‐farm oxidation ponds in New Zealand, particularly in areas where land irrigation is unsuitable.