Nobody moves to the Spanish countryside for the sewage. But the toilet is the one system you use every single day, and getting it wrong is miserable, expensive or both.
We started with a composting toilet while we worked on the house, then put in a proper septic system once the bathroom was finished. I’ve written about septic tanks versus compost toilets from the living-with-it side. This guide is about the rules: when you need a permit, why a sealed tank is treated differently, what the basin authorities accept, and what it all costs.
The rules come from Spain’s Water Act, applied by the river basin authority for your area (the Júcar, the Segura, the Guadalquivir and so on), with the town hall and regional bodies involved too. They’re broadly similar everywhere but not identical. I’m explaining the system as I understand it, not giving legal advice.
The basic rule: no discharge without a permit
The Water Act prohibits discharges that could pollute public water without prior authorisation. The bit people miss is that “discharge” doesn’t only mean a pipe into a river. The Júcar authority makes clear that it includes indirect discharge to groundwater through infiltration into the ground. In other words, the soakaway behind your septic tank.
The Segura authority’s guidance for single houses says the same: domestic wastewater can be authorised after treatment, whether it goes to surface water, to groundwater through a well, or indirectly through the soil, a filter well or a filter trench, and that discharges are prohibited without prior authorisation.
So, a classic septic tank with a drainage field is a discharge, and it needs a permit.
The exception: a sealed tank
There’s one way round the discharge permit: don’t discharge anything.
A sealed tank (*fosa estanca*) is a watertight container, usually polyethylene or concrete, that holds all the wastewater until a licensed firm pumps it out and takes it away. The Segura authority’s guidance is explicit: because there’s no discharge to public water, its authorisation isn’t needed. Two conditions: both the sludge and all the water have to be removed, and the system is assessed by the local or regional authority instead.
The catch is volume. A sealed tank fills with everything you flush, wash and shower. That means frequent emptying, and every emptying costs money and needs a truck that can reach the tank. For a weekend casita, a sealed tank can make sense. For a family living there full time, the emptying bill adds up fast, which is why most full-time homes end up with a treatment system and a permit.
| Septic tank or treatment plant with soakaway | Sealed tank | |
|---|---|---|
| What it does | Treats the water, then lets it soak away | Stores everything until it’s pumped out |
| Basin authority permit | Needed | Not needed if genuinely sealed |
| Emptying | Sludge every so often | All the wastewater, regularly |
| Best for | Full-time homes | Occasional use, or where a soakaway isn’t allowed |
What the authorities accept
The Segura guidance sets out what it expects from a single house (systems for up to 50 people are covered by the European standard UNE-EN 12566):
- Where the groundwater is highly vulnerable, a packaged treatment plant (UNE-EN 12566-3) with minimum removal rates of 70% for BOD5, 75% for COD and 90% for suspended solids, sized for at least 600 litres a day.
- In other cases, a prefabricated septic tank (UNE-EN 12566-1) with a minimum capacity of 3 m³, plus 1 m³ for every five extra people.
- A site-built system is possible, but needs a project by a competent technician showing similar performance.
The authority asks for the CE marking and declaration of performance, the product sheet and the installation and maintenance manual, a plan showing the house, the unit and the discharge point, and a description of the filtration system if the water goes into the ground. If the unit is already installed, you prove it with the invoice and photos.
A few more conditions are worth knowing. There’s no discharge permit if connecting to the municipal sewer is viable (the authority asks the town hall). Rainwater and pool water mustn’t go through the treatment system. And for discharges into the ground, the authority may ask for a hydrogeological study first.
The Júcar authority runs a similar system for most of the Valencian Community. If you’re in the Guadalquivir basin or elsewhere, ask for their equivalent guidance. Every basin publishes one.
The Valencian extras
In the Valencian Community, two more things come into play.
First, the regional sanitation agency, EPSAR, has a role in wastewater treatment, including procedures for delivering septic and holding-tank waste to public treatment plants. Your emptying company should be authorised to do that.
Second, the regional sanitation charge applies whatever happens to your wastewater. The Valencian law defines it as applying to water used in the home regardless of whether it ends up in a septic tank or the sewer. The same guidance notes that individual treatment systems that operate properly, manage their sludge and hold a valid permit can qualify for a benefit on that charge.
When a house changes hands, the town hall may get involved too. A technical architect answering a buyer’s question explained that for a second occupation licence in Alicante city, the council asks for a technician’s certificate on the septic tank’s condition and a maintenance contract with a firm that empties it, authorised to take the waste to a treatment plant. Other towns have their own lists.
What it costs
Prices depend on the system, the ground and how far the digger has to travel. These are the figures I checked this month.
| Item | Figure | Source |
|---|---|---|
| Septic tank, tank only | €900–€5,000 | Habitissimo |
| Fitting | From about €1,500 | Habitissimo |
| Licences and permits | €500–€2,000 | Habitissimo |
| Treatment plant for a house | €2,000–€4,000 | Habitissimo |
| Complete installation, one Valencian supplier | €4,000–€6,000 | Telio Homes summary |
| Emptying, Alicante guide | €134–€226, average €174 | Telio Homes summary |
| Emptying, national average | About €275 | Cronoshare |
| Composting toilet | €200–€1,500 | Leroy Merlin |
The biggest variable is the ground. Rock means a breaker on the digger and a longer job. Clay can mean a bigger drainage field. A steep plot can mean a pump. Get two or three quotes, and make sure each one says whether the permit paperwork is included.
How often it needs emptying
There’s no universal interval. It depends on the tank’s size, the technology, how many people live there and how much sludge builds up. Follow the manufacturer’s manual and the conditions on your permit, and keep every emptying receipt. If you sell, those receipts are evidence that the system works.
Access matters more than people think. If the truck can’t get within reach of the tank, emptying becomes “creative”, as I put it in my earlier piece. When you plan the system, plan the truck’s route too. My driveway saga has more on that.
Septic tank or treatment plant?
People use “septic tank” for anything buried in the garden, but there’s a real difference.
A septic tank settles the solids and lets bacteria break them down without oxygen. The liquid that comes out is partly treated, and the soil in the drainage field does the rest. It’s simple, has no moving parts and uses no electricity.
A treatment plant (*depuradora*, often “total oxidation” type) adds air, usually with a small blower, so bacteria break the waste down much more thoroughly. The water that comes out is cleaner. The trade-off is that it needs power around the clock and occasional servicing.
Which you’re allowed depends partly on where you are. The Segura authority’s guidance accepts a prefabricated septic tank where the discharge isn’t to highly vulnerable groundwater, but where the groundwater is highly vulnerable it asks for a packaged treatment plant meeting minimum removal rates. Many inland aquifers count as vulnerable, so check with your installer before you choose.
For an off-grid house, the treatment plant’s blower is a small but constant load. Count it in your solar sums, especially through winter nights.
The permit, step by step
Every basin authority has its own forms, but the process usually runs like this:
- Choose the system with your installer, based on the number of people, the ground and the groundwater.
- Collect the documents: the product’s CE marking and declaration of performance, its datasheet and maintenance manual, a plan of the house, the unit and the discharge point, and a description of the drainage field.
- Submit the application to the basin authority with the official form.
- Answer any questions. The authority may ask for more detail, or a hydrogeological study for discharges into the ground.
- Install the system (if it isn’t already) and keep the invoice and photos.
- Keep the permit and the maintenance records together, with the emptying receipts.
If you’re also applying for a licence, the town hall will usually want to see that the wastewater is solved, so it helps to start the septic paperwork early.
Where to put it
Your installer and the manufacturer’s instructions will set the details, but the principles are simple:
- Keep it well away from any well or borehole, and downhill of it if you can.
- Keep it away from the house foundations and boundary walls.
- Don’t put it where the truck can’t reach.
- Don’t put a drainage field under a driveway, a patio or tree roots.
- Avoid flood-prone ground and anywhere the water table comes close to the surface.
If you also have a borehole, read my borehole permits guide: the basin authority’s paperwork for the well asks you to map what’s around it.
Buying a finca with an existing tank
This is where most problems live. Many older fincas in inland Alicante have a tank of some sort, but it may be cracked, too small, or discharging somewhere it shouldn’t. “There’s a septic” doesn’t mean there’s a usable, legal system.
Before you buy, find out:
- What it actually is: septic tank, treatment plant, sealed tank or an old cesspit.
- Whether there’s mains sewerage nearby (rare in the campo, but it changes everything if there is).
- Where the treated water goes, and whether that discharge has a permit.
- Whether there are installation, permit and maintenance documents.
- Whether it’s sized for how the house will really be used, including guests or holiday lets.
- Its condition, from a professional inspection, ideally with the tank emptied so they can see inside.
That checklist follows the one in the Benissa guide I’ve been quoting, and it’s sensible anywhere. If the answers are bad, it’s not a reason to walk away, but it is a reason to knock the cost of a new system off the price.
Compost toilets and the law
A composting toilet deals with the toilet but not the rest: showers, sinks and washing machines still produce greywater. Rules on relying on a compost toilet as your only system vary by town hall, so ask before you build a house around one, especially if you plan to sell one day. Greywater for the garden is its own subject; see my greywater guide.
Questions people ask me
Do I need a permit for a septic tank in Spain?
If the treated water soaks into the ground or runs anywhere, yes: a discharge permit from the river basin authority, plus whatever the town hall requires for the works. A genuinely sealed tank that’s pumped out doesn’t need the discharge permit.
Is a sealed tank cheaper?
Cheaper to install and to permit, more expensive to live with. You pay to empty everything you flush and wash.
Can I keep the old tank?
Only if it’s the right size, in good condition, legal and reachable by a truck. Many older tanks fail at least one of those.
What happens if I don’t have a permit?
The basin authorities can open sanction proceedings for unauthorised discharges, and an unpermitted system can cause problems when you sell or apply for licences. It’s far cheaper to sort it out when you’re doing other works.
Who empties it?
A licensed waste firm with authorisation to deliver to a treatment plant. Ask to see it, and keep the receipts.
The bottom line
Decide early whether you’re going to treat and discharge (septic tank or treatment plant, with a permit) or store and remove (sealed tank, with a pumping bill). Then budget for it as a core part of the house, not an afterthought. When it works, you forget it exists. That’s the whole point.
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