Information

Sewage sludge processing systems in Scotland

This report on sewage sludge processing systems is part of the research project undertaken by the James Hutton Institute on the impacts on human health and environment arising from the spreading of sewage sludge to land (CR/2016/23).

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2 Regulatory and best practice approaches to sludge treatment, haulage, storage and use in agriculture

As described above, the processes of waste water and sludge treatment can take place on the same site – or can be separated, with sludges transported elsewhere for treatment. Even where both streams are treated on the same site, the objectives of the treatments differ between substrates:

1. Waste water must be sufficiently treated to reach standards that allow it to be discharged into surface waters without undue harm. Standards commonly include limits on suspended solids, biochemical oxygen demand (BOD) and phosphorus concentration; and

2. Sludges must be sufficiently treated to reach standards that allow them to be used in thermal energy recovery (incineration) or land-based markets – principally agriculture. Standards applied to sludges intended for agricultural use (as ‘biosolids’) include limits on indicator pathogens and monitoring of potentially toxic element concentrations.

This separation of objectives means that (for example) approaches that encourage hazards to partition to sludges (thereby improving the quality of the treated waste water) can impact negatively on the quality and usability of the sludges. Whilst the latter are the focus of this report, the relationships between sludge treatment (and treatability) and waste water treatment should be considered in any wider view of hazard management options.

This section focusses on sludge treatment, haulage, storage and use in agriculture – from the perspectives of regulatory requirements, good practice and the Biosolids Assurance Scheme.

2.1 Regulatory requirements

2.1.1 Sludge treatment

Hazards addressed by regulation:

  • In principle, the Sludge Directive captures all hazards relevant to ‘health’. Only PTE limits in receiving soils are prescribed How they are addressed:
    • Limits on heavy metal concentrations in soils to which biosolids are applied
    • Limits are placed on trade effluents discharged into the sewer catchment
    • Limits are placed on final effluents discharged to receiving water bodies
  • Various, depending upon the nature of sewer catchment and sensitivity of water body receiving the treated waters. pH, COD, suspended solids, phosphorus and nitrogen are commonly managed How they are addressed

The process of waste water (or ‘urban waste water’) treatment normally falls under the requirements of European Council Directive 91/271/EEC. Its objective is to protect the environment from the adverse effects of urban waste water discharges and discharges from certain industrial sectors. It covers the collection, treatment and discharge of:

  • Domestic waste water
  • Mixtures of waste water
  • Waste water from certain industrial sectors

The relationship between these types of waste water and the scope of the Directive is illustrated in Figure 2-1. Article 14 includes a requirement that sludge arising from waste water treatment shall be re-used whenever appropriate, and that sludge Disposal routes shall minimize the adverse effects on the environment (EC, 2017). No specific sludge treatments are prescribed by this legislation.

Figure 2-1 The scope of the Urban Waste Water Treatment Directive. Image from: http://ec.europa.eu/environment/water/water-urbanwaste/
This figure shows the scope of the Urban Waste Water Treatment Directive.  Image from: http://ec.europa.eu/environment/water/water-urbanwaste/

If urban waste water treatment processes handle only indigenous material, then they are not normally covered by the requirements of the Waste Management Licensing (Scotland) Regulations. However, processes that import controlled wastes such as sludges, septic tank sludge or screenings from outwith the curtilage of the works are required to hold a Waste Management Licence (WML).

The requirements of a WML will be specific to the individual works, but would normally include odour management conditions such as: ‘Waste operations shall be carried out so that offensive odours from the site as perceived by an authorised SEPA officer, do not become detectable beyond the boundaries of the site’. Depending on the scale of activity, a WWT facility may import and treat sludges under a Paragraph 10 Exemption, rather than a full WML (Quinn et al., 2016). This covers ‘Reception and treatment of specified waste at a Water Treatment Works’. Specified wastes include ‘Sludges from treatment of urban waste water’, and there is an upper limit of 100,000m3 of (combined, specified) wastes in any 12 month period. The Exemption requires that wastes are ‘managed without endangering human health and without using processes or methods which could harm the environment and in particular, without:

(a) Risk to water, air, soil, plants or animals; or

(b) Causing nuisance through noise or odours; or

(c) Adversely affecting the countryside or places of special interest

Limits are applied to treated water that is discharged back to the environment, as set out in Table 2-1.

Table 2-1 Requirements for discharges from urban waste water treatment plants. Values for concentration or the percentage of reduction apply ( HMSO, 1994)
Parameters Concentration Minimum percentage of reduction
Biochemical Oxygen Demand (BOD5 at 20°C) without nitrification 25 mg/l O2 70-90
Chemical Oxygen Demand 125mg/l O2 75

Where treated waste water is discharged into a sensitive area, then limits are placed on the phosphorus and nitrogen content of the treated water Table 2-2. A map of sensitive areas in Scotland is provided in Appendix 1.

Table 2-2 Requirements for discharges from urban waste water treatment plants into sensitive areas, which vary according to WWTW load (in population equivalent or p.e.). Values for concentration or the percentage of reduction apply ( HMSO, 1994)
Parameters Concentration Minimum percentage of reduction
Total phosphorus 2mg/l P (WWTW with loads equivalent to between 10,000 and 100,000 p.e.) 1mg/l P (WWTW with loads equivalent to more than 100,000 p.e. 80
Total nitrogen 15mg/l P (WWTW with loads equivalent to between 10,000 and 100,000 p.e.) 10mg/l P (WWTW with loads equivalent to more than 100,000 p.e. 70-80

Trade connections to the sewer network also require consent (under the terms of the Sewerage (Scotland) Act 1968, as amended) (HMSO, 1968). This makes provision for limits to be placed on characteristics of the effluent that are specific to the nature of that effluent; limits for discharges to sewer from a distillery may be very different to limits for discharges to sewer from a metal plating works.

This regulatory environment is intended to ensure that treated waste waters are of appropriate quality when discharged to the environment. They do not focus on hazards directly relevant to sludge quality. Indeed, some treatments for waste waters – such as dosing with iron or aluminium salts to precipitate phosphorus – can directly influence the quality of sewage sludge prior to treatment, and their inherent treatability. Such influences are out of scope for this report.

The European Sewage Sludge Directive (86/278/EEC) seeks to encourage the use of sewage sludge in agriculture. It prohibits the use of untreated sludges (except where injected or otherwise incorporated into soil) and defines the treatment of sludge as "biological, chemical or heat treatment, long-term storage or any other appropriate process so as significantly to reduce its fermentability and the health hazards resulting from its use”. The Directive does not prescribe treatments (EC, 2016).

2.1.2 Sludge transport

Hazards addressed by regulation:

  • No hazards specific to sludge are addressed by regulation – although loss of containment / spillage would constitute a breach of Duty of Care How they are addressed:
    • Sludge hauliers should register with SEPA as professional transporters of waste

When intended for application to agricultural land under the requirements of the Sludge (Use in Agriculture) Regulations (1989), biosolids are not considered a controlled waste, and need not therefore be transported by a registered waste carrier. However, the Biosolids Assurance Scheme (see Section 0) requires that biosolids are transported as though they are wastes. The requirement to register as a professional transporter of waste is not prescriptive as to the manner of transportation, although licensed waste carriers are subject to Duty of Care (Scottish Government, 2012). Amongst other requirements, this states that all waste holders must act to keep waste safe against:

  • Spillages from corrosion or wear and tear of containers;
  • accidental spilling or leaking or inadvertent leaching from waste unprotected from rainfall;
  • accident or weather breaking contained waste open and allowing it to escape;
  • waste blowing away or falling while stored or transported;
  • scavenging of waste by vandals, thieves or animals.

It also requires that handling of wastes avoid harm or pollution.

2.1.3 Sludge storage

Hazards addressed by regulation:

  • Odour nuisance

    How they are addressed:

    • Must not be created as a result of storing biosolids where they are to be used under a Paragraph 8(1) Exemption from Waste Management Licensing
    • Under General Binding Rules, dewatered sewage sludges must be stored in such a way that ingress of water or runoff are prevented; Field stores of dewatered biosolids must be applied to land within six months of deposition in the store
    • Sludge stockpiles must be situated at specified distances from water bodies, to reduce the potential for contamination by nutrients during storage. Within Nitrate Vulnerable Zones, long term biosolids storage areas must have an impermeable surface and must either have a facility to collect, store and recover runoff or the biosolids must be covered with waterproof covering.

The storage of biosolids where they are intended to be used (under the Sludge (Use in Agriculture) Regulations, 1989) is authorised under an exemption to waste management licensing – meaning that SEPA must be notified. Various conditions attach to this (‘Paragraph 8(1)’) exemption (SEPA, 2011):

  • the sludge is stored at the place where it is to be used;
  • the sludge is stored at a distance of not less than—
    • 10 metres from any inland or coastal waters;
    • 50 metres from any well, borehole or similar work sunk into underground strata for the purpose of any water supply other than a domestic water supply; and
    • 250 metres from any well, borehole or similar work sunk into under ground strata for the purpose of a domestic water supply;
  • no sludge is stored for longer than 6 months

It should be noted that – although such storage should be ‘secure’ – there is no minimum distance from householders or other human receptor within which such stores can be located.

Dewatered sewage sludges are subject to General Binding Rules (GBR) under The Water Environment (Controlled Activities) (Scotland) Regulations 2011 (as amended). These materials must be stored:

  • in such a way that they are securely contained so that any escape or runoff is prevented; or
  • in a heap which is protected from the ingress of water

Furthermore, if dewatered sewage sludge is stored in a heap in field, it must be applied to land within 6 months of the commencement of the storage.

As with other ‘organic manures’, biosolids are captured by the requirements of the Nitrates Directive when applied within designated Nitrate Vulnerable Zones (EEC, 1991). Within Nitrate Vulnerable Zones, long term biosolids storage areas must have an impermeable surface and must either have a facility to collect, store and recover runoff or the biosolids must be covered with waterproof covering.

2.1.4 Sludge use

Hazards addressed by regulation:

  • In principle, all hazards that might impact on the ‘quality of the soil and of the surface and ground water’. In practice, only potentially toxic elements are explicitly (if indirectly) managed

    How they are addressed:

    • Controlled by concentration limits on cadmium, copper, lead, mercury, nickel and zinc in receiving soil
  • Nutrients

    How they are addressed:

    • Application locations (proximity to water courses), timing and rate are controlled by General Binding Rules. Risk of pollution of the water environment must minimised, crop demand for nitrogen not exceeded, and soil phosphorus status maintained at ‘acceptable agronomic levels’
    • Application locations, quantities and timings for biosolids applied to agricultural land within designated nitrate vulnerable zones are restricted – under codes of practice

The principle regulatory instrument controlling the use of biosolids on agricultural land is the Sludge (Use in Agriculture) Regulations (HMSO, 1989). This requires that The sludge shall be used in such a way that account is taken of the nutrient needs of the plants and that the quality of the soil and of the surface and ground water is not impaired. In practice, the regulations limit the rate of accumulation of specified potentially toxic elements (PTEs) in soils to which biosolids are applied. Soils must be tested before biosolids are first applied, and need not then be tested again for another twenty years. However, the biosolids themselves must be tested at six monthly intervals and the ten year average loading rate for specified PTEs (in kg per hectare) calculated.

Soils must be tested for pH, chromium, cadmium, copper, lead, mercury, nickel and zinc – and soil concentrations (on a dry matter basis) are prescribed for cadmium, copper, lead, mercury, nickel and zinc according to pH. Biosolids cannot be applied to soils with pH of less than 5.0.

Spreading activities under the sludge regulations do not require prior notification to SEPA, and any odour complaints arising from agricultural spreading are normally dealt with by Environmental Health departments within local authorities (Quinn et al., 2016).

As an organic fertiliser, biosolids also fall within the scope of The Water Environment (Controlled Activities) (Scotland) Regulations 2011 (as amended). Various General Binding Rules apply to the location of spreading, which must not be:

  • Within 10 metres of any:
    • river, burn, ditch or loch, as measured from the top of the bank;
    • wetland;
    • transitional water or coastal water as measured from the shoreline; or
    • opening into any surface water drainage system;
  • Within 50 metres of any:
    • spring that supplies water for human consumption; or
    • well or borehole that is not capped in such a way as to prevent the ingress of water.

Biosolids may not be applied to land that:

  • Has an average soil depth of less than 40cm and overlies gravel or fissured rock, except where the application is for forestry operations;
  • Is frozen waterlogged or covered in snow; or is sloping, unless it is ensured that any run-off of fertiliser is intercepted (by means of a sufficient sized buffer or otherwise) to prevent it entering any river, burn, ditch, wetland, loch, transitional water or coastal water towards which the land slopes.

Biosolids must not be applied to land:

  • In such amounts that the crop requirement for nitrogen is exceeded;
  • In excess of the amount required to maintain the soil phosphorus status at acceptable agronomic levels; or
  • During heavy rainfall or where heavy rainfall is forecast within 24 hours.

The General Binding Rules also require that any equipment used to apply biosolids must be maintained in a good state of repair, and that biosolids must be applied on land in such a way and at such times that the risk of pollution of the water environment is minimised.

Requirements on biosolids’ use that derive from the Nitrates Directive (EEC, 1991) are presented in the forms of Codes of Practice, to which farmers must (voluntarily) adhere. Further information on these requirements is presented in Section 0.

Under the wider (waste management licensing) regulations, waste recovery activities (including the application of biosolids to agricultural land) must be undertaken without:

  • Risk to water, air, soil, plants or animals; or
  • Causing nuisance through noise or odours; or
  • Adversely affecting the countryside or places of special interest (SEPA, 2011).

2.2 Good Practice

2.2.1 Sludge treatment

Hazards addressed by good practice:

  • Pathogens

    How they are addressed:

    • Compliance with the Safe Sludge Matrix – to produce either Conventionally or Enhanced-Treated sludges. These require that either a 2log10 or 6log10 kill in pathogens be demonstrated, and that (for Enhanced Treated sludges), Salmonella be absent

As noted above, the relevant regulations require that (unless injected or otherwise incorporated into the soil), sludge must be subjected to biological, chemical or heat treatment, long-term storage or any other appropriate process so as significantly to reduce its fermentability and the health hazards resulting from its use. The regulations do not specify appropriate treatments. However, the Code of Practice for the Agricultural Use of Sewage Sludge (DOE, 2006) provides examples of treatment processes that may be deemed to satisfy the regulatory requirements (Table 2-3).

Table 2-3 Sludge treatment options, as listed in the Code of Practice (DoE, 1996)

  • Sludge Pasteurisation: Minimum of 30 minutes at 70°C or minimum of 4 hours at 55°C (or appropriate intermediate conditions), followed in all cases by primary mesophilic anaerobic digestion.
  • Mesophilic Anaerobic Digestion: Mean retention period of at least 12 days primary digestion in temperature range 35°C +/- 3°C or of at least 20 days primary digestion in temperature range 25°C +/- 3°C followed in each case by a secondary stage which provides a mean retention period of at least 14 days.
  • Thermophilic Aerobic Digestion: Mean retention period of at least 7 days digestion. All sludge to be subject to a minimum of 55°C for a period of at least 4 hours.
  • Composting (Windrows or Aerated Piles): The compost must be maintained at 40°C for at least 5 days and for 4 hours during this period at a minimum of 55°C within the body of the pile followed by a period of maturation adequate to ensure that the compost reaction process is substantially complete.
  • Lime Stabilisation of liquid Sludge: Addition of lime to raise pH to greater than 12.0 and sufficient to ensure that the pH is not less than 12 for a minimum period of 2 hours. The sludge can then be used directly.
  • Liquid Storage: Storage of untreated liquid sludge for a minimum period of 3 months.
  • Dewatering and Storage: Conditioning of untreated sludge with lime or other coagulants followed by dewatering and storage of the cake for a minimum period of 3 months If sludge has been subject to primary mesophilic anaerobic digestion, storage to be for a minimum period of 14 days

The Safe Sludge Matrix (ADAS, 2001) prohibits the use of untreated sludges on agricultural land, and differentiates between how Conventionally and Enhanced Treated sludges can be used for different cropping scenarios. It does not prescribe the nature of the treatment required to deliver either category of sludge, instead prescribing the required outcomes of any applied treatment:

  • Conventionally treated sludges must be subjected to a treatment that destroys 99% (or 2log10) of pathogens present;
  • Enhanced treated sludges must be subjected to a treatment that destroys 99.9999% (or 6log10) of pathogens present, and renders the treated sludge free of Salmonella.

In practice, compliance is determined by monitoring treatment impacts on E. coli, as a suitable indicator organism or proxy for ‘pathogens’. Neither the Code of Practice nor Safe Sludge Matrix provide alternatives to scenarios in which 6log10 pathogen reductions cannot be demonstrated due to low pathogen populations in untreated sludges. Suitable alternatives are instead provided under the Biosolids Assurance Scheme (see Section 0).

2.2.2 Sludge transport

Hazards addressed by good practice:

  • Odours and spillages

    How they are addressed:

    • Vehicles used for transporting biosolids must be suitable for the intended task and ‘adequately contained or covered to avoid odour nuisance’. Any spillages must be cleared-up immediately to prevent risks to water courses.

The Code of Practice for Agricultural Use of Sewage Sludge (DOE, 2006) states the following:

The movement of sludge by road tankers from sewage works to agricultural land can lead to complaints of noise and smell in built-up areas and cause serious traffic problems in country lanes.

  • To minimise the risk of creating a nuisance the type and size of vehicle should be suitable for the planned tasks. All sludge loads should be adequately contained of covered to avoid odour nuisance. Care should be taken to ensure that vehicles used to carry untreated sludge do not cross contaminate subsequent loads of treated sludge. The routes should be carefully chosen to minimise inconvenience to the public.
  • Spillage of sludge should be cleared up immediately in a manner that avoids pollution of watercourses.

It should be noted that this Code of Practice has been superseded outside Scotland (GOV.UK, 2017). It should also be noted that the requirements listed above do not apply to biosolids being transferred in an agricultural spreader from temporary field stores to the place of application – nor where treatment of sludges (e.g. using a mobile lime treatment rig) occurs at the place where the biosolids are to be used.

2.2.3 Sludge storage

Hazards addressed by good practice:

  • ‘Public nuisance’

    How they are addressed:

    • There is no requirement in the COP to manage odours during storage – whether those stores are permanent or temporary

The Code of Practice (DOE, 2006) requires that sludge storage units must be designed and constructed so that as far as practicable sludge cannot escape from them and members of the public cannot have access to the sludge stored within them. As noted above, this code has been updated and no longer includes Scotland.

Controls on temporary field stores have been established by the Water UK Biosolids Network, although are not listed in public documents. They require that, to minimise the risk of temporary diffuse pollution during the intended period of storage, biosolids cake dispatched for temporary field storage must:

  • be solid enough to be stored in a free-standing heap; and
  • not be likely to give rise to free drainage from within the stacked material

2.2.4 Sludge use

Hazards addressed by good practice:

  • Potentially Toxic Elements

    How they are addressed:

    • Code of Practice limits accumulation of molybdenum, selenium, arsenic, and fluoride in soils to which biosolids are applied
  • Odours

    How they are addressed:

    • PEPFAA requires that all odours from agricultural activities be considered and minimised, so that statutory nuisance is avoided
  • Nutrients

    How they are addressed:

    • PEPFAA, NVZ guidance and SRUC technical notes all limit rates at which biosolids can be applied to agricultural land. The condition and location of the land must also be considered before application to minimise risks from run-off
  • Pathogens

    How they are addressed:

    • The Safe Sludge Matrix prescribes harvest and grazing intervals for Conventionally and Enhanced treated biosolids in different agricultural land uses

Several codes of practice apply to the application of biosolids on agricultural land, including:

1. The PEPFAA Code (Prevention of Environmental Pollution from Agricultural Activity) (Scottish Executive, 2005);

2. Nitrate Vulnerable Zone guidance (Scottish Government, 2016);

3. The Biosolids Nutrient Management Matrix (ADAS, 2014);

4. The Code of Practice for Agricultural Use of Sewage Sludge (DOE, 2006); and

5. The Safe Sludge Matrix (ADAS, 2001).

Many of the relevant requirements of the first three codes are focussed on minimisation of water pollution risks. Thus, application of biosolids to steeply-sloping land, frozen, wet or waterlogged soils must be avoided – as must the application within close proximity of springs, wells, boreholes and other water sources. Application rates are limited according to total nitrogen concentration, while (in NVZs) applications are prohibited during winter closed periods where readily available nitrogen exceeds 30% of total nitrogen. The Nutrient Management Matrix seeks to limit phosphorus accumulation in biosolids-amended soils, to minimise long term risks of phosphorus transfer to sensitive water bodies.

Such approaches directly minimise risks from nutrient transfer – and indirectly minimise risks from other hazards present in biosolids, such as pathogens. The potential for odour during spreading activities) must be considered and managed to prevent odour nuisance. Indeed, the code covers odour potential from all farming activities.

Whilst soil concentrations for several PTEs are controlled under regulation (see Section 2.1.1), the Code of Practice (DOE, 2006) recommends limits for a further four: molybdenum, selenium, arsenic, and fluoride.

When biosolids are applied to the surface of grassland, the addition of lead, cadmium and fluoride in any one year must not exceed 3 times the 10-year average annual rates, and differentiation is made between permissible concentrations of copper, nickel, mercury and chromium and molybdenum when biosolids are applied to grassland as opposed to arable soils. Biosolids cannot be applied to the surface of grassland if their lead concentration is greater than 1,200mg/kg or their fluoride concentration is greater than 1,000mg/kg (on a dry weight basis), irrespective of ten year average loading rates.

Soil sampling depths also vary between land uses – with shallower samples required for permanent grassland (7.5cm) than cultivated land (25cm or soil depth, if shallower).

The Safe Sludge Matrix prevents the application of untreated sewage sludges to agricultural land, and prescribes the circumstances under which either Conventional or Enhanced Treated biosolids can be used (Table 2-4).

Table 2-4 The Safe Sludge Matrix ( ADAS, 2001)
Cropping category Conventionally Treated Enhanced Treated
Fruit 10 month harvest interval applies
Salads 30 month harvest interval applies 10 month harvest interval applies
Vegetables 12 month harvest interval applies 10 month harvest interval applies
Horticulture 10 month harvest interval applies
Combinable and animal feed crops
Grassland and forage – grazed Sludge must be deep injected or ploughed down; 3 week no graze interval applies* 3 week no graze interval applies
Grassland and forage – harvested 3 week no harvest interval applies 3 week no harvest interval applies
Key:
All applications must comply with the Sludge (Use in Agriculture) Regulations and DETR Code of Practice for Agriculture Use of Sewage Sludge
Applications are not allowed, except where stated conditions apply

*The matrix, as published, is ambiguous on this point – implying that there should be no grazing during the season in which Conventionally Treated sludges are applied. The interpretation presented in the table above is that of the Red Tractor Assurance Beef & Lamb Standards (Red Tractor Assurance, 2017).

2.3 Biosolids Assurance Scheme

Hazards addressed by good practice:

  • Various

    How they are addressed:

    • The BAS brings together regulatory and good practice controls, and therefore addresses the ranges of hazards described in previous sections. The BAS also introduces a requirement for Source Material Risk assessments, in which all hazards relevant to the quality of biosolids are brought into scope. In practice, the focus remains on soil PTE concentrations and pathogen kill during sludge treatment.
  • Pathogens

    How they are addressed:

    • The BAS disambiguates the situation where raw sludges contain concentrations of fewer than 6log10 E. coli, which means that 6log10 pathogen kill cannot be demonstrated by any specific process. In such circumstances, Maximum Allowable Concentrations (MAC) apply. MAC also apply to Conventionally Treated sludges, although in such cases 2log10 kill must also always be demonstrated.

Previous sections have highlighted various regulatory and good practice controls that apply to the production, transport, storage and use of biosolids. These controls have developed over a number of years to meet the changing needs of the biosolids’ market, and their diversity can be confusing. The Biosolids Assurance Scheme seeks to provide clarity by bringing regulatory and good practice controls together into a single, auditable, scheme. This unambiguously makes the many (hitherto) voluntary controls compulsory – and Scottish Water have committed to ensuring that all of their sludge treatment centres and downstream sludge management activities comply with the scheme.

Two specific areas are drawn-out in the BAS Standard that are not covered by the various regulatory or good practice requirements:

1. The application of HACCP approaches to pathogen control; and

2. The use of source material risk assessments.

These are considered below.

HACCP approaches to pathogen control

As set out in Section 2.2.1, the Safe Sludge Matrix discriminates between sludges that have been subjected to Conventional or Enhanced treatment. However, no absolute limits are specified for pathogens in the biosolids’ product. Furthermore, sludges can be treated in a variety of ways, some of which can deliver similar outcomes in terms of pathogen kill (Section 2.2.1). BAS therefore sets out a process by which samples of untreated and treated sludges are tested for E. coli to demonstrate both: log10 kill and compliance with maximum E. coli populations in the treated material:

  • For Conventional Treatment, a 2log10 kill of E. coli must be demonstrated. The treated sludge must also meet a maximum allowable concentration (MAC) of 100,000 E. coli per gram of dry solids;
  • For Enhanced Treatment, a 6log10 kill of E. coli must be demonstrated, or the treated sludge must meet a maximum allowable concentration (MAC) of 1,000 E. coli per gram of dry solids (and no Salmonella).

It should be noted that final product storage and the potential for E. coli or Salmonella populations to change during storage are not covered by the HACCP approach under BAS. Whilst process efficacy must be shown by testing samples taken before and after processing, there is no explicit requirement to test samples taken from storage.

Source Material Risk Assessment

Three different categories of source material are listed, with the extent of required risk assessment increasing as the materials diverge from straightforward urban waste waters:

1. Category A – Domestic wastewater and industrial wastewater

2. Category B – Septic tank material and water treatment sludge

3. Category C – Feedstock material (such as green waste used for co-composting with sludges)

Where Category A or Category B materials are processed, then one risk identification and risk control form must be completed for the entire organisation for each category. These forms must then be reviewed at least every thirty-six months and amended where necessary.

If Category C materials are processed, then a pre–acceptance assessment must be completed for each separate feedstock stream supplied. This assessment must be reviewed at least every thirty six months and amended where necessary. A risk identification and risk control form must be completed for each sludge treatment site that receives one or more feedstock material(s), and these forms must be reviewed and updated if there is a substantial change to the feedstock material. Where no substantial change occurs, the risk identification and risk control forms must be reviewed at least every twelve months and amended where necessary.

Copies of the various assessment, risk identification and risk control forms are provided in the BAS standard[2].

Contact

Email: gary.gray@gov.scot

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