More Output, Better Effluent Quality, From a Tight Land Package

Dec. 28, 2000
System Profile

About the author: Ian Lisk is editorial director of Water Engineering & Management and Water & Wastes Digest.

It sits there, looking out over a picturesque scene of water and tree-covered promontories, another excellent example of an old plant upgraded and expanded to handle tighter regulations and an increased population base. The vastly modified wastewater plant serving the community of Delran, New Jersey, which I visited recently in preparation for this latest article in WEM's System Profile series, is notable for the application of several technical innovations that are finding their way into design specifications more frequently as we near the middle of the third USEPA decade.

Located not many miles from downtown Philadelphia, and northwest of Camden, New Jersey, Delran Township's new wastewater facilities serve about 12,000 people at present. The plant site is very close to the confluence of the Raconcas Creek and the much larger Delaware River, both of which exhibit tidal differences of over six feet in this area. The aerial shot on the cover of the magazine shows how close the plant is to residences and a busy marina. Yet it does not seem to intrude on or be out of place in its surroundings. And not once during the visit did I get a whiff of an objectionable odor. This is just one piece of evidence of what appears to be a well-designed process scheme, coupled with the selection of processes and equipment that are getting the intended results.

In the summer of 1990 it became apparent that new facilities would have to be built by the Delran Sewerage Authority, which has responsibility for a planning area of about 7.5 square miles. The New Jersey Department of Environmental Protection called for an improvement in the existing plant's ability to reduce biochemical oxygen demand (BOD), total suspended solids (TSS), and nutrients in its discharge to the river system. It was also obvious that an increase in capacity was needed and should be worked into any upgrade project. A number of permit violations had been detected during the previous three years, mostly due, it was thought, to effluent flows that were higher than the permitted capacity.

The Authority's original plant, which could handle a wastewater throughput of 1.5 mgd, was built in the middle '60s with funds supplied from an early federal grants program. It consisted of an influent pumping station, two above-ground contact stabilization units in steel tanks, a chlorination system, and sludge drying beds, all installed on a 2.4 acre site. A sludge thickener and a belt filter press were added in the early '80s, and the drying beds were shut down.

With the plant site already limited in area, the engineers selected by Authority officials for the '90s design and construction undertaking had to develop a plan that would accomplish the expansion and the process upgrade without the acquisition of any real estate. In addition, the treatment of the township's sewerage could not be interrupted, and the discharge would have to continue to meet the mandated quality limits while the construction phases of the project were underway. A third important objective was that as much of the existing tankage on the site as possible had to be worked into the new design scheme. It certainly appears that the consulting engineers awarded the project, The Alaimo Group of nearby Mount Holly, New Jersey, in cooperation with Authority staff, contractors, and equipment suppliers, have accomplished these goals handily.

The Core Process

Considering the boundary restrictions of the site, and the requirement to make use of existing vessels and structures, the engineers proposed using a process which combines anaerobic and aerobic technologies. It was chosen after technical, environmental impact, and cost studies were carried out to investigate several possible solutions. Two important advantages became evident: the system is compact and can be housed in one suitably partitioned vessel; and projected costs for process air were significantly lower in comparison with conventional systems of similar capacity.

Also known as the biological nutrient removal (BNR) selector process, it is an activated sludge system. Particularly effective for biological phosphorus removal, with developmental roots that reach back to the early '70s, it is now offered by Davco of Thomasville, Georgia. In common with a traditional activated sludge system, the BNR selector process has a reaction section where aeration of mixed liquor takes place, a clarifier for settling of solids, and a return sludge pumping system. However, it has also a small non-aerated anaerobic zone at the start of the process, followed by a larger aerated or "oxic" zone in the remainder of the basin. Where nitrogen removal is an important consideration, an "anoxic" section is incorporated between the anaerobic and oxic zones. This is the case in the Delran installation.

The zones are subdivided into stages, as depicted in the schematic drawing. All zones and their stages are connected in series so that the mixed liquor flows sequentially from the influent feed to the weir ahead of the clarifier. Each stage of the anaerobic/anoxic section of the process is fitted with a submerged mixer. The task of this unit is to maintain gentle agitation in the compartment without causing undue turbulence on the surface of the liquid. In the oxic zone, aeration is accomplished with manifolded banks of ceramic fine-bubble diffusers.

Phosphorus removal is achieved by certain microorganisms whose growth is stimulated and maintained by the alternating anaerobic/oxic conditions. Return activated sludge (RAS) from the clarifier, which follows the last oxic stage in the process, is mixed with raw influent (which first has been passed through a comminuter and a grit removal device) in the first stage of the anaerobic zone. The RAS provides the microorganisms needed for the anaerobic reaction to proceed. Other microorganisms bring about the nitrogen removal, the oxidation which breaks down the nitrogen-containing material to nitrates and nitrites taking place in the oxic zone. Recycled nitrates are fed back to the anoxic zone for reduction to free nitrogen.

Delran's Treatment Scheme

Incoming wastewater from the collection system, which underwent some upgrade work, especially in its five pumping stations, is lifted from a below-grade wet well by new raw sewage pumps to an elevation of about 31 ft to a distribution box. On its short trip there through a 14-in., then a 16-in. pipe, it first passes through an in-pipe comminuter (a Muffin Monster), which can be by-passed if necessary. Then it is fed to an 86-in.-diameter grit removal device, known as the Teacup, which extracts 95 percent of the grit larger than 150 microns. From there it enters the elevated distribution box (see photo) which provides the head needed to maintain gravity flow through the entire plant as far as the final discharge pump station.

The distribution box feeds the three BNR units, one new and two housed in the reconstructed contact stabilization tanks. A look at the cover photo of the magazine, which was taken from the air a few months ago, will show how the newly built 150-ft-diameter reactor unit is in full operation. Beyond it the two earlier 78-ft tanks are in the midst of rehabilitation and having the Davco BNR innards installed. Each of these units has exactly half the capacity of the larger one-0.625 mgd versus 1.25 mgd, for a combined 2.5 mgd. A noticeable feature of the large system is the large surge or equalization zone, which takes up about a 100-degree section of the tank. The smaller units do not have a similar surge zone but benefit for this large one. Liquid from this section of the large unit can be recycled on demand through an automatically-actuated 14-in. pinch valve back to the influent wet well for another pass into the distribution box.

The various sections of the process are quite visible in the cover shot, as is the circular centrally-located secondary clarifier. Also visible is an externally-mounted 24-in.-diameter by-pass pipe which provides flexibility for shutting down individual segments of the reactor. Now finished, the two smaller units have the same configuration as the large reactor, but without a surge zone.

Treated and clarified wastewater proceeds from the BNR units to two tertiary rapid sand filters, each with a surface area of 540 sq ft, for polishing purposes. These are travelling bridge units with a combined capacity of 2.5 mgd at a dosing rate of 1.6 gpm/sq ft. Peak flow is 6.25 mgd at 4.0 gpm/sq ft. Each has 43 cells filled with 6 in. of gravel, 13 in. of coarse and fine sand, and 6 in. of anthracite. Backwash is done one cell at a time using the overhead bridge, so no shutdown of the entire filter is necessary.

Next stop is the ultraviolet disinfection system, a two-channel basin holding 224 UV lamps and a total arc length of just under 13,000 in. The decision to specify this advanced UV system was made with the prospect of eliminating the use of chlorine entirely at the Delran plant for the disinfection process, and consequently enhancing safety levels.

Finally, the finished effluent drops to a below-grade pumping station and is sent on its way into the river through a 16-in.-diameter outfall extending over 700 ft into the river. This has a multiport diffuser and a Tideflex pinch-type check valve at its extremity.

Sludge conditioning begins in the digester sections of the process units, which are fed wasted sludge from the secondary clarifier. Coarse-bubble diffusers were chosen for installation in the digesters to supply process air. They were less expensive than the fine-bubble type used in the oxic zones, and provide good mixing characteristics. The digested sludge is transferred to the only unit retained from the old plant doing the same job as before-the sludge thickener. At the end of it all is a belt filter press, with accessory items like a polymer feeder, and a clever conveyor set-up which diverts dried cake to either of two bays, and to different parts of the truck which takes the cake away to a landfill.

A PC-based distributed control system monitors the various processes and operations. Over 150 points throughout the plant are monitored on two computers located in the control building, including flow, temperature, pH, air flow, sludge flow and on/off status. Any changes are recorded on two printers. When an alarm is activated, a telephone dialer is used to notify the plant superintendent.

An almost final accounting of the project puts its cost at about $10,600,000, or about $600,000 less than the original bid of $11,185,000. Alaimo Group's Proj-ect Manager Steven Miller, from the firm's Mount Holly, New Jersey, headquarters, told me the savings were achieved as a result of a value engineering study, and some modifications that were made. Steve was responsible for the design, construction and start-up of the project, showed me around the plant, and provided most of the information on which this article is based.

Also accompanying me to explain things were Timothy Poole, The Alaimo Group's resident senior project manager on the site who handled construction management and field engineering, and George Conard, superintendent of the plant for the Delran Sewerage Authority. Resident Project Manager Steven Boyce with RAC General Contractors of Tabernacle, New Jersey, the prime contractor, commented about the teamwork that had existed on this job, especially since the unusually hard winter of 1994, with low temperatures and a lot of icing, had delayed much of the work by almost two months.

About the Author

Ian Lisk

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