Treatina aaueous effluents of the Detrochemical industrv The PetrochemicalIEcology Sector Group of CEFIC, the European Council of Chemical Manufacturers’ Federations with headquarters in Brussels, Belgium, finds that a central biological effluent treatment plant is the preferred method of treatment, but would cost about 10-20% of the profits of the petrochemical site it serves, resulting in a substantial loss of cashflow, which would not then be available f o r future capital investment
S. J. Nijst Shell Internationale Chemie Maatschappij B. V. Postbox 162 T h e Hague, Holland The Petrochemicals/Ecology Group created several Task Forces dealing directly with aqueous effluents from petrochemicals manufacturing. Task Force 2 studied the technical aspects of process and treatment plants and their costs. The Petrochemicals/Ecology Group defined the petrochemical industry in the form of a flowscheme of a model petrochemical site. This model was useful in limiting the number of products to those most commonly produced and setting a scale to the treatment facilities of the site. Task Force members delivered their opinion (based on their company’s performance and/or literature) on the quality and quantity of the aqueous effluents a t the battery limits of each of the process plants obtainable by reasonably good manufacturing technology. The latter implies: 652
Environmental Science & Technology
pretreatment, which is economically justified in its own right (for example, stripping and recycling of water in a steam cracker) removal of material, which impairs good operation of sewer system, etc. (for example, powder and nibs from polymer plants) removal of floating oil. Members’ data on the Raw Waste Loads (kg BOD, kg C O D and liters of effluent per ton of product) were, in practically all cases, in agreement with, or close to, those obtained by EPA in developing their guidelines for the U S . industry. It was therefore decided by the Task Force to use the EPA figures on R W L for further development as they are representative for the European petrochemical industry. The data of the Raw Waste Loads were then applied to the model petrochemical complex. Why biological treatment? The choice of treatment process for the aqueous effluent of the model petrochemical site was, unanimously, the biological process since: It is geared to BOD removal, which is generally required by the re-
sponsible authorities. Its cost to achieve a certain degree of BOD removal are low compared to other, non-biological, treatment processes. If effluent requirements were expressed in terms of COD, then a biotreater, which for petrochemical effluents will typically allow 70-80% COD removal, might be expected to be generally cheaper than the removal of the COD load in one nonbiological treatment step. Many types of biological treatment systems have been developed in the past. The systems consist of the biological reactor(s) and auxiliaries that are applied to various extents, depending on the judgment of the designer. Such auxiliaries are, for example, concentration and volume balancing tanks, suspended solids and oil removal, p H control, nutrients addition, temperature control, sludge recycle, excess sludge dewatering. Biological reactors have been applied in many forms such as: activated sludge (air or pure oxygen), biodiscs, extended aeration, trickling filters, and the like. The Task Force preferred not to
0013-936X/78/0912-0652$01.00/0 1978 @ 1978 American Chemical Society
choose a particular version of the treatment process nor to design a specific system because of the large range of possibilities, the time available, the absence of pilot plant data on the effluent in question. Instead a more general approach was taken on the basis of treatment plants already in operation (or a t the design stage) with associated cost data.
Additional treatment processes The effluent from a biological treatment system will still contain dissolved organics and suspended solids, which may be removed to a certain degree by one or a combination of several of the following processes: further biological treatment by, for example, aerated lagoons and polishing ponds sand or multimedia filtration to remove suspended solids
other physical processes (for example, reverse osmosis, ultrafiltration, and extraction), and chemical processes (for example, oxidation by ozone, hydrogen peroxide, chlorine, incineration and wet air oxidation). These processes are applied in specific cases only. General application on biotreated effluents is unlikely because of excessive costs, lack of technical experience and reliability or inability to achieve required degree of purification. The Task Force judged that of these relatively novel processes, activated carbon adsorption, preceded by suspended solids removal by filtration, would in general be the best economically available technology for reducing the residual COD of biologically treated effluents. It must be stressed, however, that little full-scale experience of this form of treatment is
60
Cyyclohexane Xylene Toluene Total
100
126 140
135
200
50 20
80
35
30
29 45
40
50
38 30
63
30
60
15
8 23 13 910
55 33 671
18
55
1 Dfl-SD.4OU.S.
Volume 12, Number 6, June 1978
653
al treatment 2
lEI 10 2
2
4 6 8
0.01
2
4
1
0.1
10
Cost, Dfl.I m3
o Individual plants(l0)
o Individual plants(12) +Model site
+ Model site
Capital investment for carbon adsorption 3
10
100
1000
(a).
1 Dfl=$0.40U.S. Environmental Science & Technology
100
Waste load throughput malh
Waste load throughput (Q),m l l h
654
Operating investmentfor carbton adsorption 4
10
1000
1 Dfl= $0.40 US.
1 Dfl=$O.
available and should therefore be regarded as the best potential process only. Literature data had to be used to develop the costs of treatment since data from member companies were scarce.
running costs (power, chemicals, labor, maintenance and overhead) C Dutch guilders per annum according to following formula:
Treatment costs Annual treatment costs = running cost capital charge. The annualization of capital expenditures is based on the use of a 10-year capital recovery factor with interest at 10% per annum, that is 0.16275. Sewer system: The investment costs for the segregation of sewers are substantial. As an extreme, BASF reported an investment for segregation, plus inplant improvements exceeding the cost of the biological treatment plant. Data from Task Force members indica'te an investment for segregation of between 20 and 50% of that for the biological treater proper. The running costs are considered to be low relative to those of the treater. Biological treatment system: Investment and running costs obtained from the open literature and from member companies were plotted against volumetric throughput and daily BOD load. Costs of sludge removal or incineration are included. BOD removal is on the average 93%. Costs are based on mid-1 976 money. Cost of land is not included. Taking into account the wide range of throughputs and BOD concentrations and, especially, the variety of designs employed, it is remarkable how well the data fit. N o better accuracy could be expected from a design made by the Task Force. Taking the throughput at Q m3/h, and the daily BOD at B kg/d then investment will be I Dutch guilders and
C = 1 1 400 Q0.55B02'Dfl/y (-20, +30%)
+
1 = 156 000 Q".'6B0.43Dfl (-30, +40%)
IDfl = $0.40 US.
Fixed runnings costs (labor, maintenance and overhead) ranged from to 2/3 of total running costs, variable costs (power, chemicals and services) being the complement. The split of running costs could not be correlated to the size of treatment plant. The costs of treatment were developed from data from treaters that had a BOD removal efficiency of around 93%. The costs of investment for other levels of BOD removal could only be derived from literature data. % BOD removal 97 93 90 80 60
Relative investment 1.45 1
.o
0.9 0.7 0.55
The effect of the degree of BOD removal on running costs is not available. Other factors having an influence on investment are: average BOD5 concentration of final effluent. A long-time average BOD5 concentration lower than about 50 to 30 m g / L may require, for example, more aeration time, secondstage aeration or removal of suspended solids by filtration. variability of BOD5 concentration of final effluent. If the probability of
occurrence of excursions to high BOD concentration is to be limited to a low value at which the ratio of that high BOD concentration to the average BOD concentration also has to be low, then measures have to be taken; for example, increased flow and strength balancing of the feed, increased aeration volume, duplication volume, duplication of equipment. The variability of effluent quality has been discussed by Frost and Conway. The Task Force members are of the opinion that an average BOD removal of 96%, as required by the US. EPA is too stringent. An average BOD removal of 93% for an effluent with an inlet BOD concentration of 500-700 mg/L results in an outlet BOD concentration of 30-50 mg/L, which reflects European practice.
Treatment costs of other processes Filtration of biotreated effluent: Mixed-media or sand filters can be used as a final polishing step to reduce solids in the biotreated effluent. Other parameters associated with the solids like BOD and C O D may be reduced as well. Flocculation pretreatment improves filtration performance. Filtration is required prior to activated carbon treatment to reduce the possibility of clogging of the columns by solids. Solids caught in the filters are backwashed, separated, and dewatered in the central dewatering facilities, which have to be designed for this additional load. From literature the following costs can be derived: Investment I = 0.02 Q0.(j6lo6 Dfl Running costs C = 5000 Qo 65Dfl 1Dfl = $0.40 US. Activated carbon adsorption: Pilot testing of an activated carbon adsorption system on an actual effluent is essential for the determination of the required contact time the adsorbtive capacity of the carbon the pressure drop across the beds the shape of the column exhaustion wave front. The adsorbtive capacity, particularly after several regenerations, has a major impact on investment and operational costs once volumetric throughput and C O D reduction are fixed. Investment costs can be split into: costs for adsorption, which are a power function of the volumetric flow rate Q (m3/h) and costs for reactivation, which are a power function of Q and carbon dosage D (kg carbon to be reactivated/m3 effluent). Volume 12, Number 6, June 1978
655
Aeration. Basin with injectors Note: carbon dosage is the ratio of C O D reduction (kg C O D / m 3 effluent) over the adsorbtive capacity (kg COD/kg carbon). Total investment I = PQ" + Q (QD)" The limited data from literature suggest that total investment will be approximately equal to I = Q0.6(0.07 0.09 Do.6) lo6 Dfl 1 Dfl = $0.40 U S . This relation is shown in the costs diagram. Variable running costs for the absorbers are proportional to Q and variable costs for the furnace are proportional to Q and D. Fixed running costs are assumed to be a fraction of the investment. So total running costs can be expressed as:
+
C = aQ
+ bQD + f I
Data from literature will give approximately C = 2004
optimistic). The total cost per m3 and per kg C O D removed are about twice those of biological treatment plus carbon treatment. The choice of treatment sequence made earlier is thus confirmed. The annual biological treatment cost for the model petrochemical complex are about 1% of the proceeds. If an activated carbon treater had to be added, the annual costs would rise to 2% of the proceeds. These costs represent 10 and 20% of profit, respectively, at a profit margin of 10% on sales, assuming that these costs cannot be passed on to the consumer. The reduction of profit will be more severe for a site that is smaller than the model complex (economy of scale). If only a proportion of the manufacturing capacity can be utilized (not uncommon these days) then the annual costs of effluent treatment will have a still greater impact.
Additional reading Chemi.-Ing. Tech., 47, No. 10, 401-413
(1975). Rizzo and Shepherd, Treating industrial waste water with activated carbon. Chem. Eng., 3 January 1977, pp 95-100. Klemetson and Grenney, Physical and economic parameters for planning regional waste water treatment systems. J . Wafer Pollut. Control Fed., 48, No. 12, December 1976, pp 2690-2699. Frost, Einhaltbarheit von Grenzwertbedingungen in Klaranlagen der Chemischen Industrie, Wasser, Luft Betr., 17, No. 6, June 1973. Conway et al., Predicting achievable effluent quality and variability in the organic chemical industry. International Conference on Effluent Variability from Wastewater Treatment Processes and its Controls, New Orleans, 2-4 December 1974.
+ 2500QD + 0.1 I Dfl/a
Model petrochemical complex The cost equations were applied to the effluent from the model petrochemical complex. Costs of pretreatment have not been taken into account as they are assumed to be carried by the manufacturing plants. However, plants that do not yet have a 'reasonably good manufacturing technology' may be faced with considerable expenditure to arrive a t the standard Raw Waste Loads. Cost figures should be estimated for each individual case. The investment in sewer system is derived from recent data for similar systems erected by member companies. These segregations were not very complicated and lengths of sewers not extremely long. 656
The influent data on biological treatment systems were: Flow: 975 m3h = Q BOD: 12 200 kg/d = B(522 mg/l) COD: 50 000 kg/d(2127 mg/L) Average conversion assumed: BOD: 93% to 36 mg/L COD: 78% to 468 mg/L In practice, tests on a pilot scale should be performed to check the assumed conversions. The BOD conversion will likely be confirmed but the C O D conversion may range from 70-90% for this type of effluent. It should be recalled that costs calculated are average costs with probable variations of 20-40% both sides. For filtration it is assumed that reduction in C O D is negligible since the inlet C O D will arise predominantly by dissolved material. A reduction of BOD from 36 m g / L to say 25 mg/L may be calculated from a suspended solids reduction of 30 mg/L with 0.4 g B O D / g solids. Influent data for the activated carbon treater were: Flow: 975 m3/h = Q COD: 456 kg/h (468 mg/L) Assuming a carbon loading of 0.2 kg C O D adsorbed/kg carbon and a C O D removal of 6.6%, a carbon dosage D = (0.66 X 0.468)/0.2 = 1.54 kg/m3 is required. Elaborate testing on a pilot scale has to be performed to establish loadings and removal. Reactivation losses were taken at 5% but may be in excess of 10%. The costs of treatment of the raw effluent in an activated carbon plant are shown in the costs diagram assuming a C O D adsorbtive capacity equal to that of the biologically treated effluent (which must be considered as
Environmental Science 8. Technology
S. J. Nijst is chairman of a task force of CEFIC (the European Council of Chemical Manufacturers' Federations with headquarters in Brussels, Belgium) that studied the technical aspects of process and treatment plants and the effect of levels of treatment on costs. This article is based on results obtained by this task force during the past two years (1976-1977). H e is an encironmental engineer in the Engineering Group of Shell Internationale, a graduate engineer f r o m Delft Technical Uniuersity, with 25 years of experience in process development and design, of which 7years are in thefield of wastewater treatment.
