Embracing the natural cycle

The focus of environmental concerns has shifted from water to waste management.

Landfill opportunities for waste disposal will reduce significantly because

of its unpopularity and regulatory restrictions. European waste policy is based

on a hierarchy of management priorities: minimisation, recycling, incineration

(with energy recovery) and landfill. Food waste, the organic fraction of municipal

solid waste, animal and sewage sludge are all biogenic materials and should

be used as a resource not treated as waste.

Damaging doubts

The alternatives to beneficial recycling are the waste disposal routes of incineration,

gasification and landfill. These pose many environmental, technical and public

acceptance problems and do not contribute to sustainable development. Despite

all of this, over the last few years, the long-term viability of the agricultural

route has been questioned and the resulting uncertainty poses a major risk to

the water utilities in their biosolids recycling operations and associated sludge

treatment processes. This in turn has a negative effect on investment in treatment

technology by water utilities and sludge technology companies who need to have

an opportunity to realise a return on their capital investment projects.

This uncertainty has arisen as a result of specific sector and general public

concerns relating to farming and food production practices in the UK. The recent

outbreaks of BSE in cattle, Salmonella and Campylobacter in poultry,

and Escherichia coli type 0157 in meat products have increased public

awareness of food hygiene and the food chain as a major route for pathogen transmission.

Whilst there is little evidence of disease in man or animals resulting from

sludge applications to land, the need for more fundamental research was acknowledged

by the industry to allow these concerns to be fully addressed.

Worldwide there is a huge amount of research into wastewater sludge and its

use on land, relating to both risks and benefits. In the US the Environmental

Protection Agency (USEPA) Code of Regulations part 503, which cover use of biosolids

in land applications, was based on an 11-year study costing $15M and covering

14 pathway risk assessments from pathogens and metals. In the UK a major research

programme was commissioned by UK Water Industry Research (UKWIR) in 1998, with

an annual budget of about £3M funded by its member water companies and

authorities plus additional funding from other sources including the Water Environment

Research Foundation (WERF), American Water Works Association Research Foundation

(AWWARF), Department of the Environment, Transport and the Regions (DETR), Ministry

of Agriculture, Fisheries and Foods (MAFF), Department of Health (DoH) and the

Environment Agency (EA).

The UK research is only a part of the work being undertaken in Europe and in

October 2002 the European Commission organised a Researching the Sludge Directive

workshop in Brussels. It was reported that the workshop found: “Overall,

the research studies demonstrate that sewage sludge can be used safely in agriculture

in the EU and contributes to the pursuit of sustainability, with minimal risk

to the environment or the food chain. Sewage sludge makes a very minor contribution

to metal levels and organic contaminants in soil and plants. The risk of pathogen

transfer to the food chain can be substantially minimised through treatment

and cropping and harvesting restrictions. Conversely, the benefits are compelling

for using the nutrient, organic matter and essential elements found in sludge

to improve soil quality and stimulate plant growth.”

The results of the UK research and other related studies were presented in

London on January 9, 2002 at the Chartered Institution of Water and Environmental

Management (CIWEM) conference, Biosolids the Risks and Benefits – an Update

on the Latest Research. Although much of the work has been separately published

this was possibly the first opportunity for so much of it to be reviewed in

public by the experts involved. Whilst most of the presentations dealt with

risk assessment, Dr Chris Rowlands in his presentation of the joint work undertaken

by ADAS, SAC, WRc and UKWIR, Biosolids Benefits to Soil Quality and Fertility,

focused on the positive benefits of biosolids use. Dr Rowlands was clear in

his view that: “With all the research undertaken and risk analysis in place,

the industry should now be taking a proactive approach rather than the conservative

and defensive position it has taken in the past, extolling the benefits offered

by biosolids.”

The authors were left with the clear view that, subject to adherence to the

Code of Practice, the use of biosolids in agriculture represents best environmental

practice in most cases, contributes to sustainable development and provides

many direct benefits to the farmer and consumer with negligible risk.

In the US, where various lobby groups have campaigned over many years against

sludge recycling, the USEPA 503 regulations which have been in use since 1993

have provided a firm scientific basis so that biosolids can be positively and

confidently marketed. Assisted by the implementation of these regulations the

percentage of facilities using sludge to land has increased from 35% in 1988

to 54% in 1997.

The US regulations classify sludge as Class A (below pathogen detection level)

and Class B (conventional) in a similar way to the new UK Code of Practice’s

enhanced and treated sludge classification. In the US there is a large asset

base in recycling technologies including anaerobic digestion, chemical stabilisation,

composting and heat drying amongst others. Already 12% of biosolids treatment

facilities in the US now produce Class A sludge and that number is increasing.

Where does that leave us? Well, the UK also already has a large asset base

in recycling technologies – mostly mesophilic anaerobic digestion supported

by thickening, dewatering and thermal drying facilities. It has long been established

that proper treatment of sludge to reduce microbiological hazards, odour, volume

and mass is essential in the production of biosolids suitable for use on land.

Existing technologies with refurbishment and enhancement as necessary, have

the capability to meet the requirements for high quality biosolids and in particular

to satisfy the new UK Code of Practice classifications for treated and enhanced

sludges, and also the requirements of the forthcoming upgrade of the European

Directive on Sludge to Land 1986.

With sensible refurbishment incorporating technical improvements existing plants

can be expected to provide many years of service at low cost with a high level

of operability and maintainability. Furthermore, existing manpower, familiar

with conventional sludge treatment processes, will also be comfortable in operating

these plants.

Combined processes

Anaerobic digestion, pre-pasteurisation, thermophilic operation and thermal

drying provide a suite of environmentally friendly modular processes which can

complement each other and be combined to meet particular site requirements.

Biological processes based on the intensification of naturally occurring pathways

are inherently environmentally friendly and minimise risk and costs. The right

combination of modular process units in the overall treatment flowsheet will

allow energy consumption to be minimised and thus contribute further to environmental

sustainability.

In the US there is increasing activity in the application of two-stage digestion,

some incorporating a thermophilic stage. This promotes pathogen destruction

and offers the prospect of enhanced process stability, gas production and solids

destruction. Others are looking at bolt-on thermal pre-pasteurisation or chemical

stabilisation, both of which can provide an immediate short-term fix. All of

these process options can be achieved with relatively low capital investment,

however revenue costs vary considerably with biological processes coming up

trumps. The lesson from the US is that the regulatory and market needs can be

addressed using existing assets. The industry needs long-term sustainable solutions

and with pressure on de-manning, robust biological processes start to sound

very attractive when faced with the alternatives.

On a broader note the industry can do more to promote recycling – a solution

which many in the water industry believe is right in the long-term. At the 65th

European Biosolids and Organic Residuals Conference, Dr Norman Lowe announced

an initiative by the EA which will investigate how partnerships can be developed

to promote recycling of organic wastes, recognising the success of the US approach.

This initiative aims to protect the practice of recycling of organic wastes

to land for the future. This will involve a change from, “you can recycle

if you wish – that is up to you and if you do these are the rules” to,

“we all think that recycling is good and we will help you to do that but

these are the rules which have to be met as a minimum”. Whilst this may

initially seem like a minor change in policy, the effect is very positive in

favour of recycling.

The practice of recycling sludge to land following effective treatment to stabilise

and reduce pathogens in line with the new Code of Practice will in most cases

provide best environmental practice and will contribute to sustainable development

in the long-term. Public confidence can be reassured with the industry taking

a proactive and positive approach to promote and provide high quality biosolids

for beneficial use.

The authors would like to thank the following for their contributions in

the preparation of this article: Dr Tim Evans, Tim Evans Environmental; Dr Peter

Matthews, EA/Monsal and Dr Chris Rowlands, Severn Trent Water.