Effect of algal recycling rate on the performance of Pediastrum boryanum dominated wastewater treatment high rate algal pond.

Recycling a portion of gravity harvested algae promoted the dominance of a rapidly settling colonial alga, Pediastrum boryanum (P. boryanum) and improved both biomass productivity and settleability in High Rate Algal Pond (HRAP) treating domestic wastewater. The effect of algal recycling rate on HRAP performance was investigated using 12 replicate mesocosms (18 L) that were operated semi-continuously under ambient conditions. Three experiments were conducted during different seasons with each experiment lasting up to 36 days. Recycling 10%, 25%, and 50% of the 'mass' of daily algal production all increased total biomass concentration in the mesocosms. However, recycling >10% reduced the organic content (volatile suspended solids (VSS)) of the mesocosm biomass from 83% to 68% and did not further increase biomass productivity (based on VSS). This indicates that if a HRAP is operated with a low algal concentration and does not utilise all the available sunlight, algal recycling increases the algal concentration up to an optimum level, resulting in higher algal biomass productivity. Recycling 10% of the daily algal production not only increased biomass productivity by ∼40%, but increased biomass settleability by ∼25%, which was probably a consequence of the ∼30% increase in P. boryanum dominance in the mesocosms compared with controls without recycling.

Digestion of wastewater pond microalgae and potential inhibition by alum and ammoniacal-N.

Algae are produced in considerable quantities in oxidation ponds, and may negatively affect receiving waters when discharged at high concentration. Thus in some instances they require removal prior to effluent discharge, which may be enhanced using flocculants such as alum. Harvested algal biomass could be anaerobically digested to methane for use as a renewable energy source, however, alum, has been reported to inhibit anaerobic digestion. Psychrophilic (20°C) anaerobic digestion experiments showed a 13% reduction in methane production with 200 g m(-3) alum in the flocculated algae, and a 40% reduction at an alum concentration of 1600 g m(-3). Elevated ammoniacal-N concentrations (785 g NH(4)(+)-N m(-3)) also inhibited algal digestion at 20°C when using an inoculum of anaerobic bacteria from a mesophylic municipal wastewater sludge digester. However, anaerobic digestion using a bacterial inoculum from a psychrophilic piggery anaerobic pond (in which typical ammoniacal-N levels range between 200 and 2000 g NH(4)(+)-N m(-3)) were unaffected by elevated digester ammoniacal-N levels and methane production actually increased slightly at higher ammoniacal-N concentrations. Thus, selecting an anaerobic bacterial inoculum that is already adapted to high ammoniacal-N levels and the digestion temperature, such as that form an anaerobic pond treating piggery wastewater, may avoid ammonia inhibition of algal digestion.

Algal production in wastewater treatment high rate algal ponds for potential biofuel use.

Wastewater treatment High Rate Algal Ponds with CO2 addition could provide cost-effective and efficient tertiary-level wastewater treatment with the co-benefit of algal biomass production for biofuel use. Wastewater grown algal biomass can have a lipid content of 10-30% of dry weight, which could be used to make biodiesel. This research investigated algal biomass and total lipid production by two pilot-scale wastewater treatment HRAP(S) (4-day HRT) with and without CO2 addition under New Zealand mid summer (Nov-Jan) conditions. The influence of CO2 addition on wastewater treatment performance was also determined. CO2 was added to one of the HRAPs (the HRAP(E)) by maintaining the maximum pH of the pond below 8. Measurements of HRAP influent and effluent water qualities, total lipid content and algal biomass production were made twice a week over the experimental period. Both HRAP(S) achieved high levels of organic compound and nutrient removal, with >85% SBOD5, >92 NH4(+)-N and >70% DRP removal. Algal/bacterial biomass production in the HRAP(E) (15.2 g/m2/d) was improved by CO2 addition by approximately 30% compared with that of the control HRAP(W) (10.6 g/m2/d). Total lipid content of the biomass grown on both HRAP(S) was slightly reduced (from 25% to 20%) with CO2 addition and the maximum total lipid content of approximately 40% was observed in the HRAP(W) when low NH4(+)-N concentration (<0.5 mg/L) and high maximum pH (>10.0) occurred. Total lipid content of the biomass increased by approximately 15% under nitrogen limiting conditions, however, overall algal/bacterial biomass production was reduced by half during the period of nitrogen limitation. More research is required to maintain algal production under near nitrogen-limiting conditions.

Nutrient removal in wastewater treatment high rate algal ponds with carbon dioxide addition.

The influence of CO2 addition to high rate algal ponds (HRAPS) on nitrogen removal was investigated using two pilot-scale HRAPs operated with different hydraulic retention times (HRT: 4 and 8 days), and was compared to the nitrogen removal by the 8-day HRT pond before CO2 addition was installed. Nitrogen balances were calculated by partitioning total nitrogen into organic and inorganic nitrogen (NH4+-N and NO3--N), and by separation of the organic nitrogen into particulate (PON) and dissolved organic nitrogen (DON). PON was further divided into algal organic nitrogen (AON) and bacteria organic nitrogen (BON) to investigate nitrogen mass flow in the HRAPS. This research shows that the proportion of algae in the algal/bacterial biomass in the longer 8-day HRT HRAP8d (55.6%) was appreciably lower than that in the shorter 4-day HRT HRAP4d (80.5%) when CO2 was added to control the maximum pH to <8.0 during the summer. Higher bacterial biomass in the longer 8-day HRT HRAP corresponded with higher nitrification rates, indicating that the longer 8-day HRT in the summer was detrimental for two reasons: lower algal productivity and increased nitrogen loss through nitrification/denitrification. Overall nitrogen removal of approximately 60% in the HRAPS with CO2 addition was mainly achieved by algal assimilation followed by sedimentation in the settling unit.

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.

Biogas recovery from a temperate climate covered anaerobic pond.

New Zealand has over 1000 anaerobic waste stabilisation ponds treating wastewater from farms and industry. Traditional anaerobic ponds were not designed to optimise anaerobic digestion to produce biogas and are therefore uncovered, releasing biogas to the atmosphere, which can cause odour problems and contributes to GHG emissions. The biogas production and treatment performance of an anaerobic piggery pond retrofitted with a perimeter cover working under field conditions was monitored over a 14 month period. The cover design proved successful in capturing biogas, mitigating odour and GHG issues and coping with New Zealand weather conditions. High solids removal rates (73% and 86% for TS and VS respectively) were achieved. An annual average biogas methane production rate of 0.263 m(3) CH(4)/kgVS(added) was observed, which is similar to gas production rates of mesophilic farm waste digesters, and indicates that the prolonged hydraulic and solids retention times of covered anaerobic ponds can fully compensate for lower operating temperatures. These results suggest that covered anaerobic ponds treating agricultural wastes in New Zealand have great potential to reduce odour and GHG emissions and recover renewable energy, while producing an easy to handle effluent for land irrigation or further treatment.

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%).

Treatment of hydroponic wastewater by denitrification filters using plant prunings as the organic carbon source.

This study investigated the feasibility of using pre-treated plant liquors as organic carbon sources for the treatment of hydroponic wastewater containing high nitrate-N (>300 mg N/L). The waste plant material was pre-treated to extract organic carbon-rich liquors. When this plant liquor was used as an organic carbon source in denitrification filters at the organic carbon:nitrogen dose rate of 3C:N, nitrate removal efficiencies were >95% and final effluent nitrate concentrations were consistently <20mg N/L. However, at this dose rate, relatively high concentrations (>140 mg/L) of organic carbon (fBOD5) remained in the final effluents. Therefore, a 'compromise' organic carbon:nitrogen dose rate (2C:N) was trialled, at which nitrate removal efficiencies were maintained at >85%, final effluent nitrate concentrations were consistently below 45 mg N/L, and effluent fBOD5 concentrations were <25mg/L. This study has demonstrated that waste plant material is a suitable carbon source for the removal of nitrate from hydroponic wastewater in a denitrification filter.

Influence of CO2 scrubbing from biogas on the treatment performance of a high rate algal pond.

Biogas produced by anaerobic treatment of wastewater can be collected and used for power generation. However, the biogas may require scrubbing to prevent corrosion by H2S and to improve engine efficiency by reducing the CO2 content. HRAP can be used to scrub biogas during the daytime when they are carbon-limited and have high pH. This study investigates the influence of the carbon dioxide addition from biogas scrubbing on high rate algal pond wastewater treatment performance (in terms of BOD, NH4-N, DRP and E. coli removal) and algal production (growth and species composition). Batch culture experiments were conducted in laboratory microcosms (2 L) and outside mesocosms (20 L). Results indicate that CO2 addition and reduced culture pH increased algal production and nutrient assimilation, decreased high pH mediated nutrient removal processes (phosphate precipitation and ammonia volatilisation), but had little influence on the ability of the culture to remove filtered BODs. Disinfection, as indicated by E.coli removal; was reduced, however, further research on virus removal, which is not affected by culture pH, is required. These preliminary findings indicate the potential to scrub C02 from biogas using high rate pond water without decreasing the effectiveness of wastewater treatment and enabling increased recovery of wastewater nutrients as algal biomass.

Biogas production from anaerobic waste stabilisation ponds treating dairy and piggery wastewater in New Zealand.

New Zealand has over 1000 anaerobic wastewater stabilisation ponds used for the treatment of wastewater from farms and industry. Traditional anaerobic ponds were not designed to optimise anaerobic digestion of wastewater biomass to produce biogas and these uncovered ponds allowed biogas to escape to the atmosphere. This release of biogas not only causes odour problems, but contributes to GHG (greenhouse gas) emissions and is wasteful of energy that could be captured and used. Biogas production from anaerobic stabilisation ponds treating piggery and dairy wastewater was measured using floating 25 m2 HDPE covers on the pond surface. Biogas composition was analysed monthly and gas production was continually monitored. Mean areal biogas (methane) production rates from piggery and dairy anaerobic ponds were 0.78 (0.53) m3/m2/d and 0.03 (0.023) m3/m2/d respectively. Average CH4 content of the piggery and dairy farm biogas were 72.0% and 80.3% respectively. Conversion of the average volume of methane gas that could be captured from the piggery and dairy farm ponds (393.4 m3/d and 40.7 m3/d) to electricity would reduce CO2 equivalent GHG emissions by 5.6 tonnes/d and 0.6 tonnes/d and generate 1,180 kWh/d and 122 kWh/d. These results suggest that anaerobic ponds in New Zealand release considerable amounts of GHG and that there is great potential for energy recovery.

Optical characteristics of waste stabilization ponds: recommendations for monitoring.

The optical character of waste stabilization ponds (WSPs) is of concern for several reasons. Algal photosynthesis, which produces oxygen for waste oxidation in WSPs, is influenced by attenuation of sunlight in ponds. Disinfection in WSPs is influenced by optical characteristics because solar UV exposure usually dominates inactivation. The optical nature of WSPs effluent also affects assimilation by receiving waters. Despite the importance of light behaviour in WSPs, few studies have been made of their optical characteristics. We discuss simple optical measures suitable for routine monitoring of WSPs (including at sites remote from laboratories): optical density of filtrates - an index of dissolved coloured organic (humic) matter, visual clarity - to provide an estimate of the beam attenuation coefficient (a fundamental quantity needed for optical modelling) colour (hue) - as an indicator of general WSP 'condition' and irradiance attenuation quantifying depth of light penetration. The value of optical characterisation of WSPs is illustrated with reference to optical data for WSPs in NZ (including high-rate algal ponds) treating dairy cattle wastewater versus domestic sewage. We encourage increased research on optical characteristics of WSPs and the incorporation of optical measures in monitoring and modelling of WSP performance.

Comparison of maturation ponds and constructed wetlands as the final stage of an advanced pond system.

The treatment performance of a maturation pond (MP), the typical final polishing stage of an Advanced Pond System (APS), is compared with that of a surface-flow constructed wetland (CW) over 19 months. Both received approximately 67 mm d-1 of wastewater after passage through upstream stages of the APS. The MP, with greater sunlight exposure, had higher algal biomass (and associated suspended solids) than the CW, showed higher dissolved oxygen (DO) concentrations and greater diurnal variation in DO and pH. Neither polishing stages reduced nutrients markedly, with the CW exporting slightly more NH(3)-N and DRP, and less NO(3)-N than the MP. Disinfection was more efficient in the MP (geometric mean 1 log load removal, 12 MPN (100ml)-1) compared to the CW (0.47 log load removal, 53 MPN (100ml)-1). Incorporation of a final rock filter (28% of area) reduced median solids levels to < 10 g m(-3) in both the MP and CW. A hybrid between MPs and CWs with alternating zones of open-water (for enhanced disinfection and zooplankton grazing of algal solids) and wetland vegetation (promoting sedimentation and denitrification, and providing refugia for zooplankton) may provide more consistent effluent quality that either stage alone.

Virus removal in a pilot-scale 'advanced' pond system as indicated by somatic and F-RNA bacteriophages.

Advanced pond systems (APS), incorporating high-rate ponds, algal settling ponds, and maturation ponds, typically achieve better and more consistent disinfection as indicated by Escherichia coli than conventional waste stabilisation ponds. To see whether this superior disinfection extends also to enteric viruses, we studied the removal of somatic phages ('model' viruses) in a pilot-scale APS treating sewage. Measurements through the three aerobic stages of the APS showed fairly good removal of somatic phage in the summer months (2.2 log reduction), but much less effective removal in winter (0.45 log reduction), whereas E. coli was removed efficiently (> 4 logs) in both seasons. A very steep depth-gradient of sunlight inactivation of somatic phage in APS pond waters (confined in silica test tubes) is consistent with inactivation mainly by solar UVB wavelengths. Data for F-RNA phage suggests involvement of longer UV wavelengths. These findings imply that efficiency of virus removal in APS will vary seasonally with variation in solar UV radiation.

Wastewater treatment by algal turf scrubbing.

Algal turf scrubbing (ATS) is a novel wetland technology that has been designed and engineered to promote natural wastewater treatment processes. Algal turf scrubbing improves water quality by passing a shallow stream of wastewater over the surface of a gently sloped floway. The floway is colonised by a natural heterogeneous assemblage of periphyton consisting of cyanobacteria, filamentous algae and epiphytic diatoms together with aerobic bacteria and fungi. Algal photosynthesis provides oxygen for aerobic breakdown of wastewater by heterotrophic bacteria. Pollutants are extracted from the wastewater by several processes including assimilation, adsorption, filtration and precipitation. The algal turf is harvested periodically to remove the accumulated periphyton biomass and associated pollutants from the system. This paper will present results from a demonstration ATS facility in Patterson, California which was used to polish secondarily treated wastewater. The design and operational factors that influence the treatment performance of ATS systems is discussed. Results indicate the potential of the ATS for nutrient removal from secondarily treated wastewater and agricultural drainage waters.

Disinfection in a pilot-scale "advanced" pond system (APS) for domestic sewage treatment in New Zealand.

"Advanced" pond systems (APS) have the potential for improving treatment, including disinfection, over conventional WSPs. Disinfection in a pilot scale APS at Ngatea, New Zealand was studied. This system comprises a high-rate algal pond (HRP) that optimises growth of settleable colonial green algae, followed by an algal settling pond (ASP) that removes much of the nutrients and solids as non-noxious algal sludge, and then a maturation pond (MP) for effluent polishing. Monitoring of this pilot-scale system over 2 years showed excellent overall removal of E. coli (average of 2000-fold reduction), with approximately 1 log removal in each of the three stages. Experiments in the pilot scale HRP suggest that most E. coli removal in this stage is inactivation by sunlight exposure, but with an important contribution from continuous dark processes. Preliminary experiments on the pilot scale algal settling pond (APS) suggest the combined effect of sedimentation of bacteria and sunlight disinfection of the (clarified) supernatant water.

Advanced pond system: performance with high rate ponds of different depths and areas.

Many domestic Wastewater Stabilisation Ponds (WSPs) or oxidation ponds in New Zealand require upgrading to reduce pollution of receiving waters. Advanced Pond Systems (APS) consisting of an Advanced Facultative Pond, High Rate Pond, Algae Settling Pond and Maturation Pond may provide a cost effective upgrade option. This paper presents the results of a 2-year study of the performance of two pilot APS systems with High Rate Ponds of different depths and areas. The HRPs of the APS systems both had the same flow rate (5 m3 d(-1)), volume (37.5 m3) and thus hydraulic retention time (7.5 d). However, the East HRP had an operating depth of 0.30 m and a surface area of 128 m2, and the West HRP had an operating depth of 0.45 m and a surface area of 85 m2. APS system performance was compared in terms of improvement of water quality. For nearly all parameters measured, there was little difference in performance between the two systems suggesting that the system with the smaller area could be used without affecting treatment. Comparison of final effluent with typical effluent of New Zealand WSPs showed that APS effluent was of higher quality and much less variable over time.

Dairy farm wastewater treatment by an advanced pond system.

Waste stabilisation ponds (WSPs) have been used for the treatment of dairy farm wastewater in New Zealand since the 1970s. The conventional two pond WSP systems provide efficient removal of wastewater BOD5 and total suspended solids, but effluent concentrations of other pollutants including nutrients and faecal bacteria are now considered unsuitable for discharge to waterways. Advanced Pond Systems (APS) provide a potential solution. A pilot dairy farm APS consisting of an Anaerobic pond (the first pond of the conventional WSP system) followed by three ponds: a High Rate Pond (HRP), an Algae Settling Pond (ASP) and a Maturation Pond (which all replace the conventional WSP system facultative pond) was evaluated over a two year period. Performance was compared to that of the existing conventional dairy farm WSP system. APS system effluent quality was considerably higher than that of the conventional WSP system with respective median effluent concentrations of BOD5: 34 and 108 g m(-3), TSS: 64 and 220 g m(-3), NH4-N: 8 and 29 g m(-3), DRP: 13 and 17 g m(-3), and E. coli: 146 and 16195 MPN/100 ml. APS systems show great promise for upgrading conventional dairy farm WSPs in New Zealand.

Combined photosynthesis and mechanical aeration for nitrification in dairy waste stabilisation ponds.

New Zealand has 16,500 dairy farms (avg. 220 cows), with cows kept on pasture throughout the year. During the 9-month dairy season, the cows are milked twice a day (averaging 2.5-3 h per day in the dairy parlour). Urine and faecal wastes deposited in the dairy parlour are washed away with high pressure hoses, using large volumes of water. A common method of treatment is in simple two-pond (anaerobic/facultative) lagoon systems, which remove about 95% of suspended solids and BOD5, but only 75% of total-N prior to discharge. High concentrations of ammoniacal-N in the effluent can cause toxicity to aquatic organisms in receiving waters. Mechanical aeration of the second (facultative) lagoon to promote nitrification improves effluent quality by reducing oxygen demand and potential ammonia toxicity to streamlife. Mechanical aeration however is associated with considerable mixing, which may prevent algae from optimising photosynthesis in the facultative lagoon. A series of experiments was undertaken which tested the efficiency of mechanical aeration and then attempted to combine it with daytime algal oxygen production in order to maximise ammonia conversion to nitrate, while minimising costs to the farmer. An experimental facility was developed by dividing a large facultative lagoon into two, producing a matched pair of lagoons, operated in parallel with influent flow split equally. Over successive dairy seasons, various aeration regimes were compared. Continuous aeration promoted nearly complete nitrification of the ammoniacal-N (99% removal), and effluent BOD was approximately halved. However the continuous mixing reduced algal biomass, and thus daytime algal photosynthesis. Night-only aeration permitted greater algal photosynthesis to occur, as well as halving electrical power consumption. Ammoniacal-N removal reduced to 90% (10 g m(-3) remaining in the effluent), while BOD removal was also lower than in the continuously aerated lagoon (59 and 69% respectively). Providing a series of biofilm attachment surfaces for nitrifying bacteria by suspending geotextile material close to the surface in the pond in consistently aerobic water resulted in improved ammoniacal-N removal efficiency (93%) with night aeration, but still lower removal than continuous aeration.

Enhancing biomass energy yield from pilot-scale high rate algal ponds with recycling.

This paper investigates the effect of recycling on biomass energy yield in High Rate Algal Ponds (HRAPs). Two 8 m(3) pilot-scale HRAPs treating primary settled sewage were operated in parallel and monitored over a 2-year period. Volatile suspended solids were measured from both HRAPs and their gravity settlers to determine biomass productivity and harvest efficiency. The energy content of the biomass was also measured. Multiplying biomass productivity and harvest efficiency gives the 'harvestable biomass productivity' and multiplying this by the energy content defines the actual 'biomass energy yield'. In Year 1, algal recycling was implemented in one of the ponds (HRAPr) and improved harvestable biomass productivity by 58% compared with the control (HRAPc) without recycling (HRAPr: 9.2 g/m(2)/d; HRAPc: 5.8 g/m(2)/d). The energy content of the biomass grown in HRAPr, which was dominated by Pediastrun boryanum, was 25% higher than the control HRAPc which contained a mixed culture of 4-5 different algae (HRAPr: 21.5 kJ/g; HRAPc: 18.6 kJ/g). In Year 2, HRAPc was then seeded with the biomass harvested from the P. boryanum dominated HRAPr. This had the effect of shifting algal dominance from 89% Dictyosphaerium sp. (which is poorly-settleable) to over 90% P. boryanum in 5 months. Operation of this pond was then switched to recycling its own harvested biomass, which maintained P. boryanum dominance for the rest of Year 2. This result confirms, for the first time in the literature, that species control is possible for similarly sized co-occurring algal colonies in outdoor HRAP by algal recycling. With regard to the overall improvement in biomass energy yield, which is a critical parameter in the context of algal cultivation for biofuels, the combined improvements that recycling triggered in biomass productivity, harvest efficiency and energy content enhanced the harvested biomass energy yield by 66% (HRAPr: 195 kJ/m(2)/day; HRAPc: 118 kJ/m(2)/day).

Investigating why recycling gravity harvested algae increases harvestability and productivity in high rate algal ponds.

It has previously been shown that recycling gravity harvested algae promotes Pediastrum boryanum dominance and improves harvestability and biomass production in pilot-scale High Rate Algal Ponds (HRAPs) treating domestic wastewater. In order to confirm the reproducibility of these findings and investigate the mechanisms responsible, this study utilized twelve 20 L outdoor HRAP mesocosms operated with and without algal recycling. It then compared the recycling of separated solid and liquid components of the harvested biomass against un-separated biomass. The work confirmed that algal recycling promoted P. boryanum dominance, improved 1 h-settleability by >20% and increased biomass productivity by >25% compared with controls that had no recycling. With regard to the improved harvestability, of particular interest was that recycling the liquid fraction alone caused a similar improvement in settleability as recycling the solid fraction. This may be due to the presence of extracellular polymeric substances in the liquid fraction. While there are many possible mechanisms that could account for the increased productivity with algal recycling, all but two were systematically eliminated: (i) the mean cell residence time was extended thereby increasing the algal concentration and more fully utilizing the incident sunlight and, (ii) the relative proportions of algal growth stages (which have different specific growth rates) was changed, resulting in a net increase in the overall growth rate of the culture.