Case Study on the advantages of a plant upgrade over a total rebuild

Cessnock Wastewater Treatment Works was built in 1936 and consisted of sedimentation, biological filtration and humus clarification processes. Unheated and unmixed digestion was employed for sludge treatment, with unburnt gas emission to atmosphere.

In the 1970's inlet works and large maturation ponds were added, and in the early 1990's an artificial wetland was built to 'polish' the effluent before being discharged.
In 2002 a study commissioned by Hunter Water showed that a major upgrade of the Cessnock plant, instead of a complete rebuild would be more cost effective and achieve significant energy savings.

The plant was undersized in terms of daily flow being received and effluent quality was variable. Mechanical equipment was badly deteriorated and screening and grit removal systems were totally inadequate. In its 2002 operational state the plant was unable to provide contemporary Workplace Health & Safety outcomes for the operators. It was also unable to satisfy New South Wales EPA environmental requirements.

EPCO Australia provided consultative services during the equipment design stage of the upgrade, prior to tenders being called. In 2005 EPCO Australia was awarded the contract to supply equipment for the upgrade of the treatment works, giving the company the opportunity to once again work with John Holland Water Group, (the head contractor for the project) with whom a mutually satisfying working relationship has been maintained over many years. The Cessnock equipment and systems sourced from EPCO Australia were delivered on time and on budget and without any lost time injuries or reportable incidents.

EPCO Australia's work scope which covered a large part of the total mechanical equipment to be installed consisted of four distinct processes, namely:

• Primary Sedimentation
• Biological Trickling Filters
• Secondary Sedimentation
• Sludge Digestion

Briefly discussing the equipment in each section in process order:

Primary sedimentation using two 16m diameter primary peripheral drive clarifiers featuring sludge and scum removal systems and the use of stainless steel construction throughout.

Primary sedimentation provides significant BOD reduction to the incoming flow due principally to removal of gross solids from the liquid stream. The collected solids are transferred to the digester whilst the clarified sewage flows to the trickling filters for the major BOD reduction stage.

Biological trickling filter mechanisms (Rotary Distributors) were retrofitted to the four existing 30.8m diameter trickling filter beds. These mechanisms feature rotating centre wells, specifically designed to provide ample stability and excellent accessibility for inspection, cleaning and maintenance. The units are operable at very low hydraulic losses.

The trickling filter is the heart of the biological oxidation process in a humus type wastewater treatment plant. The most important criteria to produce a desired effluent quality apart from having an adequate volume of growth media for the imposed flow, is the efficiency of distribution of the liquid over the growth media. This is the job of the rotary distributor mechanism.

Due to the larger than normal ratio of low to high flow required by Hunter Water, the units were fitted with primary, secondary and tertiary arms.

The spreader nozzles fitted to the distributor arms are manufactured from ABS plastic and can be customised to flow rates for each particular project. The EPCO spreader nozzle has a long history of trouble free operation over hundreds of equipment installations.

The distributors provide an even coverage of sewage over the filter area, within 90% of the outer filter radius, with a tolerance of plus or minus 5%, this degree of distribution conforms to the best results of hydraulic performance of such units.

The ends of the arms of the EPCO distributors are fitted with drainer's test plugs as the flushing gates instead of fabricated gate mechanisms. Test plugs are very simple to remove and replace, thus making arm cleaning a simple and spannerless operation.

These mechanisms were designed and developed by EPCO Australia over 40 years ago and have been subject to continuous improvement. The numerous installations still in operation throughout Australia and overseas attest to the great durability and reliability of this equipment.

One 22m diameter secondary peripheral humus clarifier provides the final BOD and suspended solids reduction stage in the main stream of the humus treatment process, principally due to the removal from the liquid stream of the solids which slough of the trickling filter growth media. The solids are recycled to the digester.

Clarified effluent flows to the maturation ponds.

Primary sludge digestion equipment incorporating:

• 13m diameter floating gas holder roof in the digester tank. The roof is fitted with manholes, sample pipes and a gas pressure/vacuum safety relief system. Additionally the roof is equipped with seal pipes into which are mounted the gas injection lances for the sequential sludge lance mixing (SLM) system. Suction pipes for the gas compressors, water heater and waste gas burner gas supplies are also mounted on the roof.

The two unique features of the roof design were:


• A requirement for no exposed structural framing. To achieve this, both sides of the structural framing were sheeted and insulation material laid between the 6mm inner and outer skin sheets during the fabrication of the cover.
• A requirement to minimise fugitive emissions of biogas from the digester. This was achieved by providing a water seal between the tank and the moveable cover.

The gas holder roof provides a stored biogas volume of 200m3 @ 300kPag thereby giving a consistent, high pressure gas supply which ensures reliable operation of downstream equipment.

• Sequencing gas lance mixing system utilises compressed digestion gas injected deep into the sludge via lance pipes. Gas mixing in the primary digestion tank is carried out to mix the incoming raw solids with the digesting sludge to maintain a homogenous sludge mass and thus optimise bacterial activity. The lance mixing system also has a universally recognised ability to prevent the formation of a scum blanket on the surface of the sludge.
  
  
• Sludge heating system comprising a biogas fired water heater, supplying hot water to a tube in tube heat exchanger capable of a 150kw heat output and a pumped sludge recirculation system.
  
  
• Waste gas burning via a waste gas flare. The flare is fitted with an automatic ignition system which ensures that the burner will automatically reignite after a flame failure or when using an intermittent gas supply. Surplus gas produced in the sludge digester is burned to reduce plant green house gas emissions. The unique burning system of our Groth range of gas flares contain flame retention vortex vanes which when coupled with the downdraft preventer in the shield, result in an updraft and an air mixing action that provides a highly efficient, smokeless and odourless burn and virtually eliminates the possibility of the flame being blown out. The wind shield is designed to control outside winds up to 300 kph, and operate efficiently in heavy rains.
  

Commissioning of the equipment was carried out progressively in conjunction with the John Holland commissioning team. Installed equipment performed to client expectations.

Results

Hunter Water have realised their expectations of the plant upgrade with the following observations:

• Over 70% reduction of greenhouse gas emissions from the plant processes. Reduction in gas emissions from the sludge digestion area in particular, methane (biogas) are almost 100%.
  
• Significant reduction of odour levels emanating from the plant.
  
• Generation of renewable energy in the form of biogas and reuse of same as heating fuel for the sludge heating system in the digester.
  
• The rate of gas production being achieved from the digester (over that required for sludge heating) is making the retrofitting of a congeneration plant a feasible option.
  
• The refined process stream now in use, coupled with correctly sized current generation equipment has resulted in significant reductions to the levels of BOD, suspended solids, total nitrogen and phosphorus in the treated effluent going to the maturation ponds.
• On site sludge dewatering is now being employed and will provide a finite reduction to plant operating expenses.
  
  
  For further information please contact Grant Cobbin on 07 3279 3276, email: grantc@epco.com.au website: www.epco.com.au