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 tan