Engineers Journal / Chemical

The river of gold inspiring a blueprint for continued innovation

Tuesday 17 April 2018

The European Union is to take action against Ireland as areas of the country have non-compliance in relation to the quality of sewer/wastewater discharges to water bodies, waterways, writes Tipperary County Council's Patrick Moran. To quote Richard Buckminster Fuller American inventor, designer and Author "pollution is nothing but the resources we are not harvesting. We allow them to disperse because we’ve been ignorant to their value". Part of one of the themes of the 2017 World Water Week programme was highlighting the symbiosis between water and wastewater to the environment and wastewater as a valuable resource to the economy.

Water and climate change

Among the biggest problems facing us today are pollution and climate change. The way we use resources and produce and use our energy lies at the heart of the effort to tackle climate change. Water and wastewater infrastructure is a significant consumer of energy and reductions in usage could be a major contributor in meeting the public sector 2020 vision of improving energy efficiency by a third. With renewable energy sources, we can cut our dependence on imported fossil fuels, reduce greenhouse gas emissions and reduce our carbon footprint. Resulting from the climate action and low carbon bill 2015 and EU renewable energy directive and C0P21, Ireland is now required to substantively decarbonise, transitioning to a low carbon economy. The country has a successful track history: in response to the oil crises in 1970 we were very successful in shifting from oil to gas and coal and, by 1990, oil use was reduced to five per cent of our primary energy source for electricity, from 65 per cent a decade before. At Tipperary County Council, achievements to date include reducing energy consumption and dependency on grid electricity; generating renewable energy; reducing costs and environmental footprint; and recycling water services sludge as a natural fertiliser. Our innovations with sludge management improves water quality, reduces the volume of sludge being removed from water services sites, and reducing truck movements, thus reducing transport CO2 emissions. There is also the potential for using biomethane from sludge digestion as a transport fuel to further reduce Co2 emissions.

Wastewater

Wastewater (used water): A combination of one or more of, domestic effluent consisting of blackwater (excreta, urine and faecal sludge) and grey water (kitchen and bathing wastewater), water from commercial establishments and institutions, including hospitals: industrial effluent, stormwater and other urban run-off: agricultural, horticultural, aquaculture, either dissolved or as suspended matter. The basic aim of this treatment is to produce an effluent (and sludge) with appropriate quality to be released to the environment or reused. More than 99.5 per cent by mass of used water consists of water, representing an enormous pool of recoverable resources. With the treatment processes available today, used water can be treated to the extent for any given reuse purpose, including recycling as drinking water, as is done in other countries.

Only about four per cent of treated water is used for drinking or cooking, the rest goes down the toilet or drain. Through the natural water cycle, the earth has recycled and reused water for millions of years but, in order to aid and accelerate this process to meet our needs, we have to work with nature using technology. At present, 40 per cent of average household demand could be met using grey water.

Used water, new water

I propose wastewater should be viewed through a new paradigm: ‘water that is wasted’. The term wastewater incorrectly gives the impression that water is considered waste when, in fact, it is used water. Accepting this realisation, the place to treat used water should be called 'used water resource recovery facilities' rather than 'wastewater treatment plants'. Used water treatment plants should be operated as energy and resource recovery facilities while improving the environment and saving money. Treated water from these recovery facilities should then be called new water.

Used water contains potentially marketable products such as biofuels, proteins, volatile fatty acids, biodegradable polymers/plastics, cellulose, construction materials, metals including precious metals such as silver, adhesives, enzymes, biofuels from algae or cooking oils etc. Let us focus on the recovery and valorisation of what is currently considered waste, by using biological resources for the production of value added products, such as bioenergy, fuels, chemicals, bio-based products fertilisers and much more. The drivers are to address the water framework and nitrates directives, sludge directive 2008/98 EC (WFD 75/442/EC), 91/676/EEC, (86/278/EEC) and EPA water research programme 2014-2020 respectively.

Used water management/treatment benefits

Managing used water is linked to the entire water cycle. Studies have shown that in terms of discharges from wastewater treatment plants they can account for a high proportion of the receiving water in winter and a higher proportion of receiving water in summer in terms of its load of solids organic matter and nutrients. Used water management treatment should be a productive process in which a desirable output (treated water) is obtained together with a series of undesirable outputs (suspended solids, heavy metals, nutrients etc using inputs(labour, energy, etc.). Used water treatment facilities are critical infrastructure for urban societies and provide essential protection for both the aquatic environment and human health. Used water management can be grouped into two general categories: market and non-market benefits. Consideration from this production process perspective also makes it possible to estimate the shadow prices of the undesirable outputs/pollutants, etc. A shadow price for these undesirable outputs is the equivalent of the environmental damage avoided if these pollutants are removed or recovered, i.e. an estimation of the environmental benefits gained from the treatment or recovery process. Environmental effects can include eutrophication, ecosystems, degradation and so on. Inadequate used water treatment can potentially lead to pollution of water bodies that are sources for drinking water. The cost of doing nothing is the shadow prices of pollutants and potential negative health effects. Therefore the discharge of used water, with inadequate treatment, involves generating significant costs, including environmental and societal ones as well as potential benefits and revenue lost. The focus is to create an environmental public good by greatly improving water quality as a consequence of used water treatment.

Costs currently associated with treatment and options

Sludge (biosolids) management is the most important cost factor in used water management and can account for up to 30 per cent of the plant's operating and maintenance costs. Treatment facilities' total operating and maintenance costs include energy, staff, reagents, waste management and maintenance. Energy and maintenance costs are a high proportion of operation and maintenance costs followed by substantial reagent costs associated with chemical dosing nutrient removal. In addition to the cost of chemical dosing, the sludge quantities produced can be increased by up to 25 per cent, thus increasing the sludge removal cost also. Less energy intensive, lower lifecycle costs are available using biological treatment systems to recover nutrients in constructed wetlands, red beds and so on. In addition, external social costs, non-monetary greenhouse gases, environmental nuisance such as noise, odours etc, should be factored in as an added benefit of resource recovery. To subsist overall treatment plants' running cost, there may also be future options to sell treated water/new water to the agricultural sector or industry among other uses.

The environmental effects of eutrophication

Biological nitrogen nutrient removal in used water treatment plants is a significant resource recovery anthropogenic source of nitrous oxide emissions with global warming potential some 300 times that of C02. Nutrients discharged have the environmental effects of eutrophication, ecosystems, degradation reduced biodiversity due to growth of algae, etc. Nutrient recovery can be divided into three sections: accumulation, release and extraction, in which nutrient products are recovered in the last step. Phosphorus is vital for plants and crops; it is limited and diminishing; and there is currently no substitute for it. Phosphorus production is anticipated to enter a long slow decline once the peak has been reached, which is estimated to occur by 2033. Phosphorus is mined from non-renewable elemental deposits and 90 per cent of the world’s phosphate reserves can be found in just five countries. Mining of phosphorus rock increases the amount of cadmium to the biosphere, which also results in severe local environmental effects. Phosphorus can be recovered from used water, urine, ash and sewage sludge. The main focus on nutrient recovery to date has been on chemical phosphorus products, which are proving very expensive, increasing sludge removal costs with no financial gain from the phosphate resource. Recapturing of used water derived phosphorus and selling it to industry seems to be a clear solution for closing the phosphorus cycle, thus protecting the environment. Crystallisation has been proven to be the established technology with the highest percentage of recovery resource for phosphorus, with a recovery rate exceeding 90 per cent and appears to be the most effective process is obtaining struvite, in which nitrogen is recovered in addition to phosphorus. The recovery of struvite also greatly reduces blockages in the plant, thus reducing operational and maintenance costs.

Some options recovery and opportunities

The cost of inorganic fertiliser continues to rise. New innovation for nitrogen recovery is via ion exchange membranes. In terms of the agricultural, economic value of the organic phosphorus and nitrogen contained in the dry sludge collected from wastewater plants, this is significant, excluding the economic value of centrates, digestates. Supernatant (digestate from digestion and centrates) could also be made available as a slow release fertiliser, and provide a potential source of revenue. Current factors such as transportation and land application are additional costs that also need to be case-studied to direct recovery options, in addition to agricultural restrictions on land spreading. Treatment of used water is a different service for different types of used water, for example, the service required by an industrial, commercial user is quite different from that of the average domestic user. Therefore, existing treatments facilities have to provide a host of services to support treatment of various influents so the services provided are not at all uniform. High-strength flow loads greatly contribute to the operation and maintenance costs of the treatment plants. As a first basic step, all industrial and commercial discharges should be tested for Ph which will help refine and guide what other testing is required in order to establish the full characteristics of the discharges. Sampling, testing/monitoring of discharges and applying the polluter pays principle is the best approach. In the longer term, once resourced, the significant users should have their used water strength tested and flow recorded on a daily basis, so as to see what the public treatment facility will be faced with treating.

Further potential resource recovery and facilities

Wastewater typically contains many times more energy than is needed for treatment. High energy from combustion of the organics in typical domestic wastewater is 6000mj/1000m3. The majority of recovered energy from used water treatment plants can be used on site in the form of electricity and heat (thermal energy) for processes. Start co-digestion using feedstock, from council sites etc as an energy crop, to feed existing anaerobic digesters, to increase production of both electricity and heat. Use of inlet screens as a resource fuel rather than disposing as a waste. Public water services sites also have potential for the installation of renewable energy technologies including micro turbines, DifGens on pipe networks, solar, wind, hydro, geothermal etc, to subsist power demand on site and export power to the grid when surplus to site demand. In the future, instead of consuming energy, wastewater treatment could also use MFCs (microbial fuel cells) and other technologies to turn treatment plants into power plants. Due to agricultural restrictions in relation to the use of wastewater sludge in the production of fruit and vegetables crops for human consumption, it is prudent to look to other alternatives outlets such as bioremediation, which is applying used water sludge on biomass energy crops, willow and miscantus, forestry, cereals for animal feeds etc. Budget 2017 allows relief from carbon tax for solid fuels that include biomass to incentivise increased usage of greener fuels. Biomass is currently the most cost-effective way to produce renewable liquid fuels. Biodiesel is one such fuel which works well in diesel engines and does not produce as much air pollution as burning petroleum fuels. Used cooking oil often thrown into sewers can also be converted to biodiesel.

Residual products

Once optimum energy and optimum resource recovery have been achieved from used water there remains marketable products. In the case of alum sludge from potable water treatment plants, one such reuse option being used abroad is in the cement industry, which will also lead to reduced C02 emissions. Author: Patrick Moran, executive area engineer, Tipperary County Council