There are 10 of thousands of species of algae. Yes, marine species have seawater chemistry in common, but freshwater algal species distribution ang growth have alot to do with the specific freshwater chemistry, e.g. Ca/Na ratios, TDS, pH etc.
You just can't lump freshwater species together, let alone grow most of them in saline condiions.

growing algae in fresh water is the best source where the same water can be recycled and used again and coming to waste water it is not most sutable for commercial purpose

RobertTulip's idea is thinking along with nature. Why arent there many enthusiasts with similar or somewhat similar ideas to cultivate algae. As Mia Franceska says, it can be faster, if only we can control such large volumes.

Constructed of large floating plastic bags of fresh water, energy from tide, wave, wind, sun and current can be combined for economical ocean based algae production to fix both peak oil and global warming, using less than 0.1% of the world ocean surface. Initial locations should be in coastal waters close to CO2 sources such as mines and power stations. Eventually it will be feasible in the deep sea, rather like robot whales. Co-benefits include reduced ocean temperature to save coral reefs and massive fish food production to solve global hunger and loss of biodiversity. The simplicity and scale of this method means we should not need genetic engineering. My proposed first step pilot is to float a ten ton ball of fresh water from Port Taranaki to Auckland in New Zealand, towed along the current by a yacht.

RobertTulips idea is bold.
http://rtulip.net/ocean_based_algae_production_system_provisional_patent

Actually that kind of an idea can stop CO2 emissions pretty quickly and completely. Instead of coastal area etc., which do not provide the scale that is required. Yes, I agree growing some algae and making some oil is better than growing nothing. But growing only in available coastal belt is rather an insignificant amount of algae.

The disadvantages of SEAWATER is that algae cannot grow as well in seawater. The salt will, in many cases, instantly kill the algae being grown. That is why the best option is fermented droppings from either cows, or horses.

I agree with Larsyn.

Sea water and coastal belt for algae cultivation is going tobe difficult because of lack of scale.
There arent enough land available and many coastal areas are inhabited. Waste water is industry specific or not large enough available at one place.
But waste water does convert the water to good and grows algae at thesame time - althoug this too doesnt solve the problem of scale.
May be we should look at low cost high volume pbrs. !! ie low cost large volume pbrs. is it practical !!?

The use of sea water to produce algae in quantity will only expose the world to a great risk of GE algae escaping into a uncontrolable ocean enviroment.

I think technology can come to the rescue. There are RO machines designed in such a manner that their membranes can't clog with impurities and go bad. We have a number of others as well. It all depends what you're going to do with it post algae growth.

ALOHA ALL,
If one is to grow alga to "burn" as a fuel, then waste-water technology is favored. The "pharmaceutical-grade" alga produced in my PBR is, by an order of magnitude, much too valuable for it's constituents to "Burn" as a fuel replacement.

Well, I think the only place that salt water will work well enough is in the ocean. I can see using the tide levels as a pump to move the levels of working algae media in containment 'cells' that are constructed of curtains made of reinforced plastic sheets; weight on the bottom and floats on the top. What do you think?
Alan

I think growing algae in organic manure is the best idea. Although many people think that horse/cow manure are superior, my research has proven the efficiency of human manure.

Only the ocean has the scale of area, resources and power to grow algae with the efficiency and scale sufficient to replace fossil fuels.

My ideas on this are futuristic but practical: http://rtulip.net/ocean_based_algae_production_system_provisional_patent