Monday, April 26, 2010

Increasing the Efficiency of Aeration in Aerobic Wastewater Treatment

Aerobic wastewater treatment uses microorganism to feed on waste in the water and convert them to sludge, carbon dioxide and water. To keep the process going, the wastewater needs to be aerated with oxygen. There are many types of aerators used for this purpose, but all of them involve huge input of energy. Aeration in wastewater treatment is a very energy intensive process. One small wastewater treatment for a PE (Population Equivalent) of 80,000, for example, consume 2,400,000 kWhr. (Malaysian Ringgit 600,000 or USD169,671.26) annually just for aeration. Increasing its efficiency even if only by a small percentage may make a significant difference.

Wouldn't it be great if one can have an aerator that can add oxygen to the wastewater without great "external input of energy"? What is this "Zero Energy Input" Waterwheel Aerator all about? I know this "Zero Energy Input" thing will raise some eyebrows! Essential, what it aims to do is to make the energy intensive water aeration more efficient. What I am refering to is a waterwheel aerator using the EXISTING surplus kinetic energy present in wastewater treatment systems to power the aerator to squeeze more oxygen into the wastewater. Thus there is no need for expensive additional input of energy. In oxidation ditches, excess flow velocity is detrimental to the oxygen transfer rate and need to be slowed down. In fact, it seems baffles are introduced to slow down the velocity:

"Where power input intensity is high, for example when treating strong wastes, the induced velocity in the ditch will also be high and the differential speed between mixed liquor and rotor blades reduced, causing a loss in aeration efficiency. This may be overcome by installing deceleration baffles upstream of the rotor. A steel channel or timber beam submerged in the top layer of the ditch and spanning the ditch width may be used" (M A Pells "Oxidation Ditches in Wastewater Treatment" p209).

So if it is necessary to slow down the velocity, why not use it to power a waterwheel to squeeze more oxygen into the wastewater?

The device (Rotafall) being proposed consists of circular discs fitted with suitable number of paddles. When flowing wastewater pushes against the paddles, the discs rotates. As the discs rotate, wastewater clinging to the surface of the discs are brought up and large surfaces of the wastewater are constantly exposed to the air for the diffusion of oxygen into the water. Tests shows that rotafall can add about 3.3 to 4 kg oxygen per square meter surface area per annum. A plate has 2 surfaces, so per square meter of plate it can add 6.6 to 8 kg oxygen to the water. To further increase the efficiency, the plates can be colored black to absorb solar energy. This will increase the temperature of the bacteria culture sticking on the plate, and will increase in exponential manner the rate at which they uses up the oxygen in the wastewater, and thus a lower dissolved oxygen concentration in the wastewater. A lower concentration will lead to a higher oxygen transfer rate.

The concept is sound, the device put waste surplus kinetic energy to work, it works. Only question is, can one construct the waterwheel aerator cheap enough, or will the cost of energy increase enough to justify the amount of additional oxygen added to the wastewater.

Some wastewater treatment plants use diffusers which produces rising air bubbles to aerate the wastewater. Thus, another version of Rotafall is feasible. This one will use the buoyancy provided by the rising bubbles trapped in the paddles to power the rotation of the waterwheel.

Update 15 June 2007

To date, I have obtained only 6 comments in this post to date, some of which were spam comment and had to be deleted. There were no comment on whether this innovation is workable and viable. Trying to get more feedback. How to do it? Found someone asking a question What is Carbon Dioxide. I saw 2 answers there, one of them which was very good.

So I decided to join BlurtIt and ask a question related to this post. Signing up was easy. I then proceeded to ask a question, but found they have a limit to the number of maximum 100 words, and there is a word count which didn't seem to reflect the actual number of words. Anyway I proceeded to type my question including a hyperlink to this post which has a long URL as shown below:

I proceeded to type the HTML including pasting the URL in, but couldn't see the end of what I entered into the question box. So what I did was to edit the hyperlink HTML so that it just consist of the above URL minus the <a href=".... I couldn't see or access the end of the question and posted it anyway, but it was actually published with the full URL visible, but not clickable. You can see the question here: Is this a workable solution to increasing aeration efficiency for wastewater treatment?. I wasn't happy that the URL was not clickable.

So I tried to resort to using and redoing the question. I really can't remember fully what I did, but I do remember observing that the end of my question still wasn't visible or accessible. I think I went ahead to test it anyway, tried to change the question to "Is this a viable solution to increasing aeration efficiency for wastewater treatment?". The response was "This question has been asked before...." even though I changed the title of the question. Now just in case my inability has anything to do with asking a question more than once, etc., I opened a different browser and signed in again. (This is one advantage of using more than one browsers which I have often recommended to bloggers in my post Why you should be using more than one browsers. I still don't understand why so many surfers are sticking to just one browser Internet Explorer 6 when there is one highly recommended and superior browser What is Firefox and why you should be using it, which offers the very useful tabbed browsing and other advantages.

Anyway, I signed in to BlurtIt again in a new browser and proceeded to ask the question, this time using the short URL that generated for me. This time, I prepared the message in MS Word with the following question with HTML as shown in the scrollbox below:

Can I get feedback if <a href=>Increasing aeration efficiency</a> is viable?

I did a word count and came up with the following result:

Words: 11
Characters (no spaces): 91
Characters (with spaces): 101

The last one exceeded 100, so I really don't know if I would succeed in publishing the question. Anyway I proceeded to copy and paste it into the question box, and I think the end of the question still wasn't visible or accessible. I clicked "Ask my question now" and the question actually got published with a clickable link to this post. You can see the question at Feedback on an innovation please.

Let's see if I get a response there or some additional feedback in the comments here. Also, I see a "Help" link at BlurtIt and I will take the trouble to ask them why I am having difficulties in seeing or accessing the end of the message and if anything can be done. Will update this post if I managed to contact them and get a response