6.0 Neutralising acid leachate and acidic water

Aglime is the cheapest neutralising agent and is generally not harmful to plants, livestock, humans and most aquatic species. The limitation of its application is its insolubility in water, although it is more soluble in strongly acid water. Using aglime to increase the pH of water can be slow and costly.

More soluble neutralising agents such as sodium bicarbonate NaHCO3 are quick to act and not subject to pH overshoot. Other cheaper, fairly soluble neutralising agents include hydrated lime (Ca(OH)₂) and quick lime (CaO) but they are difficult to manage and can result in excessively high pH. When using these strongly alkaline materials, strict protocols must be established for their safe use, handling, monitoring and their effects on the receiving environment

Soluble or caustic neutralising agents such as hydrated lime Ca(OH)₂ that has a pH 12 can quickly increase the pH and should be used with caution. Overdosing natural waterways with hydrated lime can impose environmental risks similar to acid conditions, with the potential to damage estuarine and wetland ecosystems. It should be noted that when neutralising acid water, no safety factor is used. However, the monitoring of pH should be carried out regularly during neutralisation procedure and for a suitable period afterwards to verify the appropriate pH has been achieved and maintained.

6.1 Calculating the quantity of neutralising agent for acidic water

The rate of application will vary with the solubility, the fineness of the neutralising agent, the application technique and the pH of the water. As a general guide, Table 5 shows the minimum quantities of pure aglime, hydrated lime and sodium bicarbonate needed to treat impounded water or drains of 1 megalitre (ML)(1000 m³) capacity. The calculations in this table assume that low saline water acidified by hydrogen ions (H+) and does not take into account the considerable buffering capacity or acid producing reactions of some acid salts and soluble species of aluminium and iron.

As ASS drainage normally contains many acidic ions capable of producing acidity by hydrolysis (e.g. Fe³⁺, Al³⁺), a sample should be taken for laboratory analyses (for measurement of titratable acidity) to more accurately determine lime requirements. If this is not available, at least a field titration using the water and the proposed ameliorant should be undertaken to provide an estimation of dosage rates.

Table 5: Quality of pure lime neutralising agent required to raise from existing pH to pH7 for a megalitre of low salinity acid water

Current water pH	[H+] (mol/L)	H+ in 1 ML (mol)	Aglime to neutralise 1ML (kg pure CaCO2)	Hydrated lime to neutralise 1 ML (kg pure CaCO2)	Sodium bicarbonate to neutralise 1 ML (kg pure NaHCO2
0.5	.316	316,228	15,824	11,716	26,563
1.0	.1	100,000	5,004	3,705	8,390
1.5	.032	12,000	1,600	1,185	2,686
2.0	.01	10,000	500	370	839
2.5	.0032	3,200	160	118	269
3.0	.001	1,000	50	37	84
3.5	.00032	320	16	12	27
4.0	.0001	100	5	4	8.4
4.5	.000032	32	1.6	1.18	2.69
5.0	.00001	10	0.5	0.37	0.84
5.5	.0000032	3.2	0.16	0.12	0.27
6.0	.000001	1	0.05	0.037	0.08
6.5	.00000032	.32	0.016	0.012	0.027

Notes on Table 5:

1. 1 m³ = 1000 litre = 1 kilolitre = 0.001 Megalitre
   
   
2. Correlations between current water pH and [H+] (mol/L) do not account for titratable acidity. The titratable acidity component should be included in any calculations of neutralising agent requirements.
   
   
3. Agricultural lime has a very low solubility and may take considerable time to even partially react. While aglime has a theoretical neutralising value of 2 mol of acidity (H+), this tends to be only fully available when there is excess acid. This, together with it's very low solubility, means that much more aglime beyond the theoretical calculation will generally be required.
   
   
4. Hydrated lime is more soluble than aglime and hence more suited to water treatment. However, as Ca(OH)₂ has a high wer pH, incremental addition and thorough mixing is needed to prevent overshooting the desired pH. The water pH should be checked regularly after thorough mixing and allowing sufficient time for equilibration before further addition of neutralising product.
   
   
5. Weights of material given in the table above are based on theoretical pure material and hence use of such amounts of commercial product will generally result in under treatment.
   
   
6. To more accurately calculate the amount of commercial product required, the weight of neutralising agent from the table should be multiplied by a purity factor (100/ Neutralising Value for aglime) or (148/ Neutralising Value for hydrated lime).
   
   
7. If neutralising substantial quantities of ASS leachate, full laboratory analysis of the water will be necessary to adequately estimate the amount of neutralising material required.
   
   
8. Neutralising agents such as hydrated lime Ca(OH)₂ , quick lime CaO, and magnesium oxide MgO neutralise 2 mol of acidity (H+), while sodium bicarbonate NaHCO₃ and sodium hydroxide NaOH neutralise only 1 mol of acidity.

• the quality and purity of the neutralising agent being used;
  
  
• the effectiveness of the application technique;
  
  
• the existence of additional sources of acid leaching into the water body that may further acidifying the water;
  
  
• the neutralising agent has become lumpy and sinks to the bottom of the waterbody, rendering it ineffective; and
  
  
• the neutralising agent becomes coated with insoluble salts, reducing it effectiveness.

Neutralisation may be faster if higher rates are used, but this is not recommended, as it is expensive and resource wasteful. Moreover, overdosing may result, though this is less likely to be a concern with agricultural lime.

To increase the efficiency, the neutralising agent should be mixed into a slurry before adding.

6.2 Methods of application include:

• spraying the slurry over the water with a dispersion pump;
  
  
• pumping the slurry into the waterbody with air sparging (compressed air delivered through pipes) to improve mixing once added to water;
  
  
• pouring the slurry out behind a small motorboat and letting the motor mix it in;
  
  
• incorporating the slurry into the dredge line (when pumping dredge material); or
  
  
• using mobile water treatment equipment such as the ëNeutra-millí and ëAqua Fixí to dispense neutralising agents to large water bodies.

In some circumstances a neutralising agent in its solid form can be used, for example by:

• placing it in a porous bag of jute or hessian and tying the bag to drums so that it floats in the water. The material will then gradually disperse. This technique should only be considered where there is significant water movement; or
  
  
• passing water across a bed or through a buffer of coarsely ground limestone CaCO2 or other granulated neutralising agent. However, this is unlikely to be effective in the long term as coarse particles of the neutralising agent may become coated with insoluble iron or other compounds, washed away or dissolved.

When the pH of ASS leachate is below 4.5, it usually contains soluble iron and aluminium salts. When the pH is raised above 4.5, the iron precipitates as a red-brown stain/scum/solid, which can coat plants, monitoring equipment, the floors or walls of dams, drains, pipes, piezometers and creeks. In addition, the soluble aluminium is a good flocculent and may cause other minerals to precipitate or for suspended clay particles to flocculate.

Where the water contains considerable soluble iron, large quantities of acid can be generated as the pH is raised and iron hydroxides are precipitated. It is important to let any sludge settle before using treated water (otherwise it will block pipes and pumps) or before discharging treated water (to avoid adverse aesthetic and environmental harm). Chemicals can be used to reduce the settlement time if it does not settle quickly enough for the staging of the works, however care should be taken in choosing flocculating agents as these can also alter pH or cause other environmental impacts.

The large-scale dosing of waters to alter the chemical characteristics, such as may be the case in the mining industry, is a specialised and highly technical task that requires considerable expertise and experience. Professional guidance should be obtained in these situations.

The pH of the water should be checked daily during the first two weeks following application or until the pH has stabilised and then on a regular basis according to the ASS management plan. The pH should be checked at least daily if there is any discharge from the site and preferably more frequently depending on the environmental sensitivity of the receiving environment.