Author: Patrick Moran, executive area engineer, Tipperary County Council In September 2011 due to inadequate sludge processing and high sludge removal costs from wastewater plants I turned my focus to see how this could be addressed for plants in my functional engineering area. Previously, operation of sludge holding tanks on sites resulted in watery, low, dry, solid sludge being produced and insufficient holding processing of sludge in the plants, with a knock-on effect from the plant's final discharge to the receiving river. I engaged my team in planning the construction and operation of new sludge dewatering beds with innovative decanting doors and weather-resistant breathable covers. We then engaged with John Mooney T/A Lisheen Engineering in the detailed design and construction of this sludge management infrastructure on our wastewater sites. The infrastructure created varies slightly from site to site governed mainly by process control and individual sludge characteristics. Our previous accomplishments included reducing waste and pollution, increasing water quality, and reducing energy usage, with works such as:

  • Construction of intake works and automated screen installation to wastewater plants and rag catchers to pump stations which have improved effluent quality and greatly reduced blockages;
  • Contracted a company and project managed it carrying out robotic cleaning of live reservoirs which meant continuity of water supply could be maintained throughout and leaks in the reservoir structure could be identified due to the head of water, which were then later repaired. This robotic technology eliminated the need for staff to enter a confined space;
  • Implemented and had carried out a programme of unidirectional scouring of public drinking water networks;
  • Managed the rehabilitation of water boreholes supported by CCTV surveys;
  • Managed the electrical modernisation of Roscrea wastewater plant, got our combined heat and power plant certified and signed a contract with the ESB for payment for electricity we are exporting from the combined heat and power plant running on sludge biogases;
  • Implemented energy-efficiency improvements, resulting in reduction in electrical consumption in water and wastewater plants.

Why dewatering beds as the solution?

These beds are simple to operate and energy efficient, providing the least-cost technology option for efficiently dewatering sludge. The operation and maintenance costs have to be considered as a labour cost although this work is carried out by full-time caretakers who also perform various other duties at these plants.

The dewatering beds

These beds were constructed of reinforced concrete to hold wet heavy corrosive sludge. The bed floors have sharp cross and longitudinally falls to decanting doors.

Decanting doors

The new beds were fitted with self-bleeding stainless doors designed with slots at intervals to mimic the old laths decanting system, but operating to continuously bleeding the water at all levels from the sludge. These decanting doors were manufactured in stainless steel due to its corrosion resistance and pliability which aid their cleaning by tapping with a hammer or water jet. Recognising Ireland has very high annual rainfall and therefore the typical problem is that the efficiency of the traditional sludge dewatering beds decreases as a result of rainfall inputs, we decided to cover the beds which have proven to produce sludge with higher dry solids, resulting in further reduced sludge volumes.

The frame for dewatering bed covers

Following materials review the decision was made to have the frame for covers to the drying beds fabricated in stainless steel, due to its corrosion resistance and strength to weight ratio making them maintenance free, durable and easy to lift when required.

Dewatering bed covers

Resulting from materials review the decision was made to have the bed covered with black water resistant breathable woven polypropylene fabric. The black cover means all light rays pass through it creating heat under the cover which aids evaporation of water from the sludge. The black woven polypropylene fabric is lightweight strong, breathable, water resistant, durable and flexible so transpiration of the moisture can also occur through the cover yet keeping the rain off, and requiring no maintenance. [caption id="attachment_23760" align="alignright" width="300"]atipp2 Borrisoleigh wastewater plant new dewatering beds with decanting doors and covers[/caption]  

Supporting strong economy

In late 2011 I selected Roscrea wastewater plant as a sludge hub for the smaller plants in the eastern half of North Tipperary. Roscrea was selected as a hub because it has a sludge digester linked to a combined heat and power plant with excess capacity to harness and convert the gas from sludge, including sludge imported from our satellite plants, as identified by prior volatile test results for sludge from these plants. Roscrea wastewater plant is exporting electricity to the national grid from its combined heat and power plant system running on sludge biogases. Our combined heat and power (CHP) has been certified, an agreement has been signed with ESB Networks and a smart meter was installed on our electrical supply so as to receive payment for the electricity we are exporting. Sludge transport savings and income from exported electricity should act to subsidise Irish Water's operating costs, so savings should be passed on to the public. Since importation of the sludge the use of calor LPG to assist in heating the digester has reduced, resulting in a significantly reduced spend. Sludge biogas generated by the digester has risen, resulting in increased CHP running hours and the generation of electricity for more hours each day. Surplus electricity above what is used on site is exported to the national grid, generally during night-time hours.

Impact on quality of life of the community

Wastewater continues to affect our lives even after it disappears down the drain. Wastewater treatment has the potential also to affect health, the local economy, recreation, utility bills, taxes and other aspects of everyday life. Laboratory results show that since the installation of this sludge management infrastructure final effluent performance from these plants has improved regarding discharges to the rivers from these plants. There is also the local benefit for children on school trips, engineering students, other local authorities and Irish Water staff visiting these plants as they are acting as a practical demonstration of innovation in sludge management etc, thus planting the seed of learning, creative thinking and future innovative initiatives in the community.

Impact on quality of environment

Reduced nuisance and greater recreation on uncongested roads reduced dust, noise, Co2 emissions, less damage to road surface, better road safety due to decreased truck movement as a result of greatly reduced number of loads of sludge being removed from sites per year. Results:
  • €200,000+ saving annually to the public due to greatly reduced sludge volumes needing to be removed off sites, as the new infrastructure increased typical sludge dry solids from previously 0.4 per cent to eight per cent thus greatly reducing sludge volumes;
  • Significantly reduced spend on LPG gas at Roscrea wastewater plant due to replacement with biogases from sludge;
  • Reduced electricity bills at Roscrea plant due to generating more of our own power on site;
  • Irish Water to receive revenue for electricity we are exporting to the national grid from Roscrea plant.
  Our achievements feed into the three strategic themes of our corporate plan:
  1. Strong economy. Our improvements encourage and support business growth in the local economy around these plants; the reduction of waste; reduced spend on LPG; and electricity at the Roscrea plant; significant financial savings; revenue from electricity generated and exported to the grid.
  1. Quality of life. Better public health due to less noise and vibration; less damage to roads and better road safety due to deduced truck movements. Visits to these sites provide practical learning opportunities promoting creative thinking and innovation initiatives in the community.
  1. Quality of environment. Improved air quality reduced truck volumes Co2 emission and improved river water quality from discharging wastewater plants.

Learning, credits, future

The infrastructure provided has made a significant contribution to the economy through the design and delivery to the public service financial savings via innovation and lean business initiative, creating process efficiencies. I am proud of what we have achieved and must acknowledge my team, some who have recently retired. Their assistance and diligence in engaging and embracing this new infrastructure was paramount in making it the success it proved and continues to be. This success is a major recognition of the outstanding work undertaken each day by the team. We in water services are most fortunate to have such creative, motivated and committed office and outdoor staff. We are currently working on developing sludge as a product, not a waste.