, -

is

Industrial wastewater from the center, which sitson 147 acres adjacent to San Francisco Bay, is pretreated to remove oil

Reducing the number of and suspended solids before being dis-

water treatment plant (WWTP) . charged to the airport’s industrial waste- solvent=based Cleaners

ered the site’s heavy metal discharge lim- its in 1987, the maintenance center was re- quired to upgrade its pretreatment

and detergents, which

resulted in greater loading processes. At the time, copper, lead, cadmium and nickel were the center’s principal con-

cyanide and detergents also were factors. cerns, although chromium, phenol, on the wastewater More recently, toxicity to fish and other aquatic organisms has become an issue. treatment system.

To address these concerns, the main- tenance center decided to minimize waste, isolate wastestreams and improve pretreat- ment processes. Specifically, the strategy included:

eliminating cyanide stripping so-

batch treating and recovering lution wherever possible,

metals from spent plating solu- tions, reducing chromium and hydrox- ide precipitation, batch treating phenol and metal- bearing paint-stripping wastes, and reducing the frequency of air- craft washing.

At the same time, the center also focused on reducing atmospheric emissions of volatile organic com- pounds (VOCs) , primarily from sol- vents. However, reducing the number of solvent-based cleaners increased the use of water and detergents, which resulted in greater loading on the wastewater treatment system. The center also installed a 0.6 million- gallon stormwater retention basin to intercept the first half inch of runoff from the aircraft parking areas and washrack. Collected runoff is routed to the oil-water separation plant.

of-pipe treatment needs in 1988 by performing source monitoring and characterization studies to identify major pollutants and their sources. (This information also was used for

The center began addressing end-

waste minimization planning.) These sources included all major

sumps and wet wells in the industrial sewer system, the aircraft washrack, stormwater holding basin, plating shop sumps and the paint-stripping wastewater treatment unit.

Analyses were performed for heavy metals, oil and grease, suspended solids, pH, phenol, cyanide and detergents. eab le 1 shows the maximum daily con- centration allowed for each constituent, the number of samples exceeding the standard, and the range and average val- ues of those constituents.) Average dry- weather flow is 300 gallons per minute (gpm), with a peak wet-weather factor of two.

The key heavy metals were cadmium, chromium, copper, lead and nickel. Oil and grease were not a problem, although the existing oil-water separator eventually

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July/August 1994 29

was replaced with a larger unit.

sequent studies involved testing specific equipment to iden- *processes that would meet the discharge limits.

lect iron sulfide precipitation to precipitate or coprecipitate heavy metals.

mium, chromium or nickel. Membrane and ionexchange processes also were rejected because they required extensive preconditioning and were not cost-competitive.

Process development. Bench-scale studies verified that sulfide would precipitate the metals, but using flocculation and sedimentation to remove fine precipitate posed a signifi- cant problem. Inorganic coagulants and various cationic, an- ionic and nonionic polymers were tested until a cationidan- ionic combination was found that worked.

A onemonth pilot test using a solids contact reactor/clari- fier unit was performed to evaluate process performance and obtain on-lime design data. The unit was rated at 100 gpm for

Besides laboratory and bench-scale treatability tests, sub

A screening process eventually led the project team to se-

Hydroxide precipitation could not meet the limits for cad-

water treatment applications but could handle only 50 gpm for sulfide precipitation because of the light floc that formed.

Four problems were identified during pilot tests: uncontrolled discharge of batch-treated paint-stripping water, which contained high concentrations of heavy metals and phenol, caused process upsets; wastewater temperature variations caused thermal strat- ification and short-circuited treatment units; flow and waste variability interfered with consistent per- formance; and nickel was the critical heavy metal, even when every- thing was working well.

After investigating these problems, the project team con- cluded that the treatment process for paint-stripping waste water required better control, equalization was needed to over- come composition and temperature variations, and nickel would be a continuing problem. It also decided to place the fil- ter units ahead of the carbon contactors to minimize plugging.

Team members then visited two comparably sized indus- trial WWTPs that use sulfide precipitation processes. These

T A B L E 1

Oil Separation Plant Effluent Constituent New Standard Number of

Daily Maximum Samples Statistics for Samples Exceeding Standards Concentration Exceeding

Standard (mg/Ll Average (mg/L)

I Metals I I I I I Chromium I 0.1 1 I 0.13-0.80 I 0.32 t I Copper I 0.20 I 7 I 0.22-0.3 3 I 0.26

0.19 ~ 1 0.06-0.48 1 I k i d 1 0.056 I 10 - I - I Mercurv I 0.001 I 0 I I Nickel ' I 0.071 I 28 1 0.12-0.77 I 0.32 I Silver I 0.023 I 0 I I

I Nonmetals I I I I

1 ' I 10.9* I 10.9 1 Phenol I 1.5 I 7 I 1.7-6.2 I 3.7 - 1 - j oil &Grease 1 120 l o I I Settleable Matter I 0.5 I 0 I I

Notes:

Semimonthly 24-hour composite samples, from December 1986 through January 1988. Number of analyses is 28. Caused by spill.

MBAS: Methylene blue active substance; anionic surfactants that react with methylene blue to form a blue chloroform-soluble complex; the intensity of color i s proportionot to concentration.

1

30 July/August 1994

The new metals removal plant, shown here near the end of construction, began operating in 1992.

visits confirmed both the viability of sulfide precipitation and the value of flow equalization.

The culmination of this testing and research effort is a sys- tem that includes screening, flow equalization, dissolved air flotation, and heavy metals precipitation using ferrous sulfide in a reactor/clarifier. The plant has a nominal operating ca- pacity of 300 gpm, with a peak hydraulic capacity of 700 gpm.

Effluent is filtered through anthracite coal and partially treated with granular activated carbon (GAC) . Oily sludge is dewatered in a rotary drum vacuum filter and incinerated; sul- fide sludge is dewatered in a filter press; and metal sludge is disposed in a Class I landtill or hauled to a smelter for metals recovery.

Pilot tests and the original system design were based on a process developed for and patented by the US. Air Force, but high royalty charges for a process license led the project team to select a similar but less costly alternative.

Specifically, the team was interested in the suliide precipi- tation stage of Permutit’s Sulfex process, in which ferrous sul- fide is injected into the wastestream along with cationic and anionic polymers to precipitate, flocculate and settle out heavy metals. @he first stage of the process - hydroxide

T A B L E 2

San Francisco Metals Removal Plant Performance Summary Constituent Permitted Daily

Concentration Maximum Influent Effluent

(mg/L) Range (mg/L) Average (mg/L) Range (mg/U Average (mg/L)

Metals

1 Arsenic I 0.20 1 0-1.4 1 .07 I 0-.02 I .005 I 1 Codmium I 0.03 I 0.2 - 1.9 I .24 I 0-.04 I ,008 I

Cksp-qivm 0.1 1 Copper 0.20

Mercury 0.001 bad 0,056

N&Id, 0.07 1 Silver 0.023

0.58

I Nonmetals I I

[ Phenol I 1.5 I I 3-46 1 18 I I 17-380 I 130 1 Oit&Grease I 120

Settleable Matter I 0.5 0 - 3 8 6.0 0 - .05 .02 Notes:

These data are based on semimonthly 24-hour composite samples taken from June 1992 through May 1993 (24 samples).

*MBAS: Methylene blue active substance; anionic surfactants that react with methylene blue to form a blue chloroform-soluble complex; the intensity of color i s proportional to concentration,

I I

July/August 1994 31

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The pilot system featured a Densadeg metals precipitation unit manufactured by Infilco-Degremont (Richmond, Va.). The unit contains three compartments: a two-stage, back- mixed tank, a recirculating reactor containing a turbine mixer, and a cladier/thickener with tube settlers.

precipitation - was ineffective on the wastewater's low ini- tial metals concentrations.)

Typically, the sulfide precipitation stage occurs in a solids-blanket reactor/clarifier, but because of space re- strictions, the maintenance center's process is a hybrid that uses the second stage of the Sulfex process with a modified equipment configuration and flow regime.

Instead of the Sulfex solids-blanket reactor/clarifier, the system features a Densadeg unit manufactured by Infilco- Degremont (Richmond, Va.). This metals precipitation unit is a threecompartment apparatus consisting of a two-stage, back-mixed tank, a recirculating reactor containing a tur- bine mixer, and a clarifier/thickener with tube settlers.

The plant is fully confined in a containment basin to protect against accidental spills. Instruments measure flow, pH, turbidity, siudge blanket level and streaming current. All process units are computer monitored and controlled, with dual programmable logic controllers and dual terminals. Special covers, vents and flame arresters were installed on the inlet works, equalization tanks, dis- solved-air flotation unit and oily sludge storage tank to comply with new air emission regulations for VOCs.

Operating experience. After the first year of opera- tion, several process changes were made:

July/August 1994

Ferrous sulfide batching difficulties caused by chemical impurities were resolved with metered batching and the use of noncaking granular fer- rous sulfate. GAC plugging caused by bioslime was reduced with periodic caustic soda treatment. Foaming caused by the wastewater's high detergent levels was reduced by better controlling washrack oper- ations and adding water sprays at foam-producing locations. Toxicity of the undiluted effluent was reduced by switching to non- toxic detergents and cleaners. Nickel discharges in excess of treat- ment capability are being addressed by tighter material inventory control, source controls and reduced use of cyanide and other chelating agents.

spills in the main facility were ad- dressed by improving communica- tion among the center's various operating areas.

Process upsets from accidental

Efforts are continuing to improve 'plant performance, reduce chemicals costs and isolate sources of problem contaminants that can result in process upset or cause the plant to exceed its discharge limits.

tory (Table 2). Occasions when discharge limits have been exceeded typically involved a short period of time and were the result of an uncontrolled discharge in the center. The system reduces total heavy metal concentrations (nine met- als) 91 percent, fi-om 4.1 milligrams per liter to 0.36 mil- ligrams per liter.

Initial characterization and treatability studies were performed in 1988, the first year of a five-year drought. As water conservation efforts were implemented, con- stituent concentrations increased, so when the treatment plant began operating in 1992, it received a more concen- trated waste than anticipated.

During the winter of 1992-1993, an above-average rain- fall thoroughly flushed accumulated solids from the collec- tion system and greatly increased the mass loading on the plant. As a result of these changing conditions, operating the facility has been a continuing challenge. As operations return to a more predictable routine, performance should become more reliable and economical. E

Overall process performance has been generally satishc-

Daniel F. Seidel is a senior project manager at Kennedy/Jenb Consultants (Sun Francisco). Neil Denton is a staffengineer at the United Airlines Maintenance Oper- ations Center at Sun Francisco International A i e o ~ t .

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July/August 1994 33