DISCUSSION

L. L. Krohn, Union Oil Co.: Have you made an analysis of the off-gases from that furnace?

Roger Zanitsch: Yes, we have. We've looked at in excess of 100 different industrial waste carbons. We've analyzed the off-gases before and after afterburning on many of these, to determine what temperature and residence time is needed for organic destruction.

L. L. Krohn, Union Oil Co.: Do you have any feel for the particulate matter coming out?

Roger Zanitsch: We have systems operating that are designed with a 2 second residence time which is primarily there to consume the particulates. If you have a half second residence time which is certainly sufficient to destroy most of the organics present, you'll still have some carbon fines that will need to be scrubbed. We feel that the 1-2 second residence time at 1800°F can destroy the carbon fines as well as the organics.

L. L. Krohn, Union Oil Co.: Considering the new source review to build this system?

Roger Zanitsch: Reactivation furnaces are now in operation and many are being designed for industrial waste applications. Technology exists to handle essentially all air pollution control requirements at a reasonable cost.

Mac McGinnis, Shirco, Inc.: We have made some of the economics that you're talking about for our electric regeneration furnace and compared them with similar economics as you have presented here from multiple hearth and other approaches and just a couple of comments a couple of factors that we have included that you haven't mentioned are in the area of utilities, scrubber water which on small capacity units may be a fairly significant contribution of operating costs; and the other factor you mentioned quality of the product, laboratory labor, lab time to confirm that the product is indeed of the desired quality, can be a fairly significant contribution.

Roger Zanitsch: I'm glad you brought this up. In the analysis that I showed, the operating cost included a 10% general plant service allowance on the total operating cost to cover overhead items such as accounting, quality control, etc. As far as scrubber water cost and disposal, it can be a factor. In those installations where we have scrubbers, we've recycled water through the pretreatment system to remove the carbon fines. Frankly, we haven't found this to be a significant cost factor.

DISCUSSION

Mac McGinnis, Shirco, Inc.: Well, a half cent here and half cent there, it begins to add up. The other general comment is you've indicated that there is considerable data on regeneration costs in multiple hearth furnaces in particular and you've showed us some trend lines in terms of direct operating costs. Can you comment on any specific data, you know, accumulated over a period of time that indicates an actual cost figure for some specific ap-. plication?

Roger Zanitsch: The numbers which I presented are based on our experience in operating both small and large furnaces.

Mac McGinnis, Shirco, Inc.: One last comment would you say then that the actual data would fall within that plus or minus 20% about your nominal curve?

Roger Zanitsch: On industrial waste applications, yes. In process applications, such as the decolorization of sugar solutions, operating costs are substantially lower since they have a constant feed and a very predictable product.

Colin Grieves, Amoco Oil Co.: First, would you care to comment on some of the new technology which you eluded to? And second, would you like to say anything about regeneration of powdered activated carbon?

Roger Zanitsch: As far as the new technologies are concerned, I was personally thinking of the electric furnace and the fluidized bed furnaces. The Japanese have several different types of furnaces. Most of the experience with the newer furnaces has been in either pilot-scale or on the commercial scale, but in considerably less corrosive application than you have in industrial wastes. In industrial waste applications, the big awakening has been in the areas of corrosion, maintenance costs, and feed interruptions. The new technologies have not been demonstrated in this type of service. As the new technology develops, it's going to take some time to gain the experience necessary to apply these new furnaces in the industrial waste effort. As far as powdered carbon activation, I don't really feel qualified to discuss it on the basis that I would only be expressing my opinions since no commercial experience has been developed.

"ACTIVATED SLUDGE ENHANCEMENT: A VIABLE ALTERNATIVE TO TERTIARY CARBON ADSORPTION"

Leonard W. Crame

Senior Chemical Engineer, Texaco Inc.

INTRODUCTION

In view of the possibility of more stringent 1983 BATEA (Best Available Technology Economically Achievable) effluent guidelines, 1,2,3,4 petroleum refiners are faced with the dilemma of an insufficient data base to determine the proper approach for making cost-effective improvements. The EPA previously proposed granular activated carbon adsorption after activated sludge treatment as BATEA technology; however, the current emphasis is to consider both effluent quality and the cost effectiveness of attaining the desired results. Two proposed approaches to BATEA technology are (1) increasing the sludge age (or mean cell residence time) of the activated sludge biomass to develop a more diverse population capable of assimilating biorefractory organics or (2) adding powdered carbon directly to activated sludge aeration basins. Both alternatives to tertiary carbon adsorption would require little capital investment and would lower operating costs.

Grutsch and Mallatt5,6,7,8,9,10,11 have proposed that the best refinery end-of-pipe treatment for soluble organic removal should include pH control, equalization, optimized dissolved air flotation (DAF), and high sludge age (20-50 days) activated sludge treatment. High sludge ages (SA) require mixed liquor solids levels above conventional levels (5-10 days SA). These higher levels increase solids flux and must be considered in secondary clarifier solids loadings. Also high effluent TSS, despite less frequent sludge wasting, can result in a loss of mixed liquor solids.

Grutsch and Mallatt emphasize that optimized chemically-assisted DAF pretreatment (or comparable pretreatment) reduces the colloid charge (zeta potential) to maximize particle agglomeration for efficient flotation, and reduces the organic load on the activated sludge unit (ASU). Removing colloids normally present in raw refinery wastewater allows better bioflocculation and lower effluent total suspended solids (TSS) since most refinery colloids and biosolids have repelling negative charges. The microbial population could then acclimate to the biorefractory organics by producing enzymes which reduce these to simpler biodegradable substrates. Current reports from within the petroleum industry seem to indicate some benefits for increasing SA. Other investigators12 have reported that high SA (low food/microorganism ratio) produces poor sludge settleability.

As a result of pilot studies at the Du Pont Chambers Works, Hutton and Robertaccio13 were issued a U.S. patent14 for the Du Pont PACT process.15 The PACT process basically involves the addition of powdered carbon (or fuller's earth, etc.) to an ASU, usually in a range of 50-400 mg/1 based on influent flow. Du Pont has reported 16,17,18 a number of advantages of the PACT process which include:

(1) color removal,

(2) stability against shock loadings,
(3) improved BOD removal,

(4) improved refractory organic removal,

(5) resistance to toxic substances,

(6) improvement in hydraulic capacity,

(7) improved nitrification (mainly in municipal wastes),

(8) foam suppression, and

(9) improved sludge settling and increased clarifier capacity.

A disadvantage of the PACT process is that the system can become very expensive if powdered carbon addition rates become high (hundreds of mg/1), even though powdered carbon is cheaper than granular carbon.

DeJohn and Adams19,20,21 have developed a considerable amount of pilot study data on activated sludge-powdered carbon systems. They report significant enhancement in studies involving refinery and petrochemical wastewaters. DeJohn and Adams explain the powdered carbon enhancement mechanism as localization and concentration of oxygen and pollutant as the result of adsorption on carbon surfaces, resulting in a more complete biooxidation. The adsorption of biorefractory organics allows a longer residence time for these components in the system. Other researchers22,23 have found similar improvements using activated sludge-powdered carbon systems and propose analogous enhancement mechanisms.

Rizzo24 has reported a case history of a full-scale activated sludge-powdered carbon demonstration run at the Corpus Christi, Texas, Sun Oil refinery. Results included better system stability, reduction of foaming, resistance to upset conditions, lower effluent suspended solids and clearer effluent, and improved organic removal. These improvements were achieved by maintaining only a 450-mg/1 powdered carbon reactor concentration with a 10-mg/1 powdered carbon dosing requirement. The shortcoming of this investigation was that a parallel control could not be run simultaneously and most improvements reported could possibly have been attributed to better clarification.

The merits of powdered carbon enhancement have been further confused with the more recent development of several types of powdered carbons with significantly different properties.