Dear Benjamin W. Portner

Usually it is somewhat risky to dispose of this water without treatment. You have to take a look at (1) your local environmental agency / regulations; (2) the microalgal species you intend to culture: if there is a non-native (exotic) species among the species you plan to use, you should never let any residual water untreated, or you run the risk of introducing a species that may become invasive (i.e., have undesirable ecological and/or economical impacts) in the new environment; and (3) your levels of nutrients and any other compounds you use to keep your microalgae growing and healthy (e.g., antibiotics).

- Is N and P concentration low enough? How much is acceptable?

It depends on the baseline conditions of the waters in which your microalgal culture wastewater will be disposed of. You must also comply with any regulations for your country / state / municipality concerning these levels and also of other compounds used in culture.

- Could high NaCl concentrations in the cultivation medium be potentially harmful to sea life?

Only if your salinity differs enormously from the salinity of the water body receiving the wastewater. If both have typical seawater salinity (or somewhat similar) you should have no detectable effects, but this depends on the volume of wastewater and the volume of the receiving waters as well.

- Could algae left in the waste water pose a risk, e.g. could they set foot in the sea and cause an artificial algal bloom?

Yes, in particular if you are using different species from the ones naturally occurring in the area, and if your wastewater has more nutrients than the receiving water body. Harmful algal blooms are known to occur in response to cultural eutrophication (i.e., artificial nutrient enrichment).

The good side is that you can use your own microalgae culture to get rid of exceeding nutrients.

Some references that may be helpful:

https://www.google.com.br/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=2ahUKEwjhl5OA95TdAhUBvZAKHZ_NAIoQFjACegQIBxAB&url=https%3A%2F%2Fwww.researchgate.net%2Fpublication%2F257517484_Microalgae_and_wastewater_treatment&usg=AOvVaw2SMAdH8Lahvso_gkYhKgU8

Article Microalgae and wastewater treatment

Article Optimizing microalgae cultivation and wastewater treatment i...

Dear Benjamin

I think Yes.

Industrial and large-scale production of biofuels from algae may cause water pollution.

Biofuels can be produced from micro-algae grown in ponds or tubes on land. This requires considerable fertilizer inputs that may pollute the environment if not treated properly. To ensure sustainable production of biodiesel from micro-algae, it is important to develop cultivation systems with low nutrient losses to the environment.

Bernardo Antonio Perez Da Gama

Thank you for this very helpful answer. Many good points that I haven't considered before (similiarity to naturally-occurring algae in the region, nutrient baseline in the sea water). Thank you also for the references which I will have a closer look at! If you have more points, let me know here or via pm.

Kenneth M Towe

"Presumably pilot-plant studies have provided some answers" Presumably. If you know any studies, please let me know.

"But, it goes further than cultivating algae because what happens to those algae also has consequences." That is definitely true and will be part of my analysis.

Mohammad Mehdizadeh

Thank you for your answer. The nutrients are an important point. I found many studies focussing on N and P recovery wih algae. Do you happen to know any studies which focus on nutrients besides these two?