Microbac Ltd

Adapter bacteria control oil spill in bulk

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Courtesy of Microbac Ltd

Introduction

Microbac was called in to advise on a required treatment for an integrated bulk oil handling facility and sea-land terminal, receiving, storing and shipping a wide variety of liquid fuels, inorganic chemicals and petrochemicals, many of which are toxic to microorganisms. The variable and complex waste streams from tank truck cleaning, ballast and tank cleaning discharges from ocean-going tankers and ground water run-off which may contain significant amounts of spilled chemicals, place unusual demands on the terminal's 2000 MT/day wastewater treatment facility. The original system, installed in 1973, proved inadequate. It was extensively modified in 1976 to handle heavy oil capacity during periods of heavy run-off.

Influent

Influent to the treatment system comes from two marine bulk liquid storage terminals, one on-site tank truck terminal and several other tank truck terminals that transport their wastewater to the treatment plant.

On-site influent results from cleaning of hoses, pumps or pipelines used during a product move or storage, from cleaning tanks before changing product, from hydrostatic tank testing, from product spills and from wastewater generated during operation of the terminal's boilers and sanitary facilities. The quantity and chemical loading of the wastewater are determined by the cleaning requirements, housekeeping and spill control effectiveness.

Wastewater is generated at the tank truck terminal by washing of product residue from internal compartments, engine and body washing, boilers and sanitary facilities. The products handled include acids, caustics, oils, petrochemicals and inert materials shipped in bulk tank truck quantities. Each product group may require special cleaning methods which, coupled with the varying load requirements, contribute to a fluctuating hydraulic and chemical load in the waste treatment system.

Waste Treatment Facility

A schematic of the modified treatment facility is shown in Figure 1. The various influent streams pass through a contaminated sewer to a lift pump station. During dry weather, two centrifugal pumps transfer the waste stream through an API separator to a neutralisation basin, from which it is pumped to the primary equalisation tank.

During wet weather three high-capacity centrifugal pumps transfer the waste directly from the lift station to a primary equalisation tank at a rate designed to minimise flooding and possible overflow of the contaminated sewers into the non-contaminated sewer system. One of the two oversized equalisation tanks installed during system modification is usually left nearly empty to accommodate unexpected overflow conditions.

From the primary equalisation tank the water is batch pumped daily to the secondary equalisation tank, from which it passes through a flow controller to the primary air flotation unit. After air flotation, to remove a majority of floating oil and grease, the waste is pumped to the bioreactor units from which it flows by gravity to a 250 MT centre feed clarifier.

From the clarifier the waste is pumped through a coagulant-polyelectrolyte rapid mix tank and flocculation basin to a second dissolved air flotation unit and finally to the effluent lift station where it is monitored before being pumped to the outfall.

System Flexibility

The normal operating flow pattern can also be altered to suit changing conditions. Flow configuration include constant feed to the aeration basin without effluent discharge; bypass of one or more process units; and segregated flow loops to handle spills, eg. a loop including primary equalisation-API-neutralisation-DAF-primary equalisation is used for oil spills. This system flexibility permits continued efficient operation during period of upset, mechanical breakdown, maintenance and unusual influent conditions.

Sludge from the air flotation units and 'wasted' sludge from the final clarifier is received in sludge storage tanks, from which it is transferred to an aerobic digester and eventually applied to a land farm where bioaugmentation is also applied. The proportion of clarifier sludge returned to the extended aeration basin controls system biological activity.

Adapted Bacteria

During four oil ballast transfers which occurred in rapid succession, the system experienced severe organic shock loadings. The combined COD from the ballast transfers was approximately 1.4 times the monthly average loading, while oil and grease levels were 1.2 times the monthly average. During these increased loadings, mechanical malfunctions in the primary dissolved air flotation unit allowed oil and grease to accumulate in the aeration basin. Treatment efficiency was lowered below acceptable limits.

To restore system performance, it was decided to seed the aeration basin with bacterial cultures having a higher affinity for hydrocarbon that the existing populations. MICROBAC H, a dried adapted bacterial culture specifically designed to degrade toxic and inhibitory hydrocarbons, was chosen. The product was added at the inlet to the bioreactor units.

While the addition was underway, a batch reactor was set up in the laboratory containing the same influent and seeded at the same rate.

Results

Oil and grease degradation in the batch reactor is shown in Figure 2. A 61% reduction occurred within 12 hours after seeding, 81% after 24 hours and 95% after 72 hours.

The batch reactor data correlated with results at the bioreactor unit outlet (Figure 3). Oil and grease degradation of 89% occurred within 24 hours. Over the next ten days, the bioreactor units showed an average effective removal of 77%.

At the start of the treatment, oil and grease levels in the aeration basin had reached 500 to 600 mg/l. After 7 days, mixed liquor and grease levels dropped below 100 mg/l and after nine days, the level was reduced to 7 mg/l.

During the period of MICROBAC H addition, mixed liquor volatile suspended solids (MLVSS) levels dropped from about 5000 mg/l to average of 2500 mg/l, while the system sludge volume index and alkalinity of the effluent increased. This was accompanied by the formation of a scum layer on the clarification. These symptoms were at first attributed to possible nitrification/denitrification in the system but laboratory tests showed no substantial amounts of ammonia or nitrate/nitrite. The effects apparently were due to the enhanced ability of the MICROBAC H bacterial strains to compete for the available organic substrate. The new microorganisms in effect displaced the existing microorganisms in the system, creating a 'new' sludge. The ratio of volatile suspended solids (VSS) to total suspended solids (TSS) remained stable throughout the 10 day period and overall system performance was not adversely affected. The sludge wasted from the clarifier and skimmed off the top of the clarifier and the dissolved air flotation unit was stored in a surplus bulk liquid trailer until levels in the aerobic digester had been reduced enough to accommodate them.

The additional oil degradation due to the introduction of adapted bacteria provided an immediate solution to a biological process overload. The higher degradation rates not only permitted the plant to maintain compliance with discharge standards but also gave the indigenous bacterial population time to adapt to new influent conditions. MICROBAC H proved an effective aid to the operation of the treatment system under shock loading conditions and now serves as a safety measure, like the recycling povisions to prevent discharge of insufficiently treated effluent.

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